Meeting people at a dinner
Listen to introductions at a dinner party to practise and improve your listening skills.
Do the preparation task first. Then listen to the audio and do the exercises.
Preparation
See moreListening A1: Meeting people at a dinner – preparation
Choose the TWO correct answers. One answer is not correct.
Transcript
See moreCharles: Hello, Julian.
Julian: Hello, Charles. How are you?
Charles: I’m fine. Fine. Julian, do you remember Alyssa?
Julian: No, I don’t.
Charles: She’s Ben’s sister. Do you remember? We were all at Ben’s wedding together.
Julian: Ah, yes, I do – in that old castle. Was it in January?
Charles: Yes, it was! Alyssa was there.
Alyssa: Hello. Nice to meet you … again.
Julian: Hello, nice to meet you too, Alison.
Alyssa: I’m not Alison.
Julian: What?
Alyssa: My name isn’t Alison. It’s Alyssa.
Julian: I’m sorry. Nice to meet you, Alyssa.
Charles: Good. Would you like a drink?
Julian: Good idea.
Alyssa: Yes, please.
Charles: Here you are.
Alyssa and Julian: Thanks!
Task 1
See moreListening A1: Meeting people at a dinner – 1
Put the sentences in the order that you hear them.
Task 2
See moreListening A1: Meeting people at a dinner – 2
Are the sentences true or false?
Hydrocarbons
In this section, you will review the structure and names of hydrocarbons. As you may recall from your previous chemistry studies, hydrocarbons are the simplest type of organic compound. Hydrocarbons are composed
entirely of carbon and hydrogen atoms, and are widely used as fuels. Gasoline, propane, and natural gas are common examples of hydrocarbons. Because they contain only carbon and hydrogen atoms, hydrocarbons are
non-polar compounds.
Scientists classify hydrocarbons as either aliphatic or aromatic. An aliphatic hydrocarbon contains carbon atoms that are bonded in one or more chains and rings. The carbon atoms have single, double, or triple bonds. Aliphatic hydrocarbons include straight chain and cyclic alkanes, alkenes, and alkynes. An aromatic hydrocarbon is a hydrocarbon based on the aromatic benzene group. You will encouter this group later in the section. Benzene is the simplest aromatic compound. Its bonding arrangement results in special molecular stability.
Alkanes, Alkenes, and Alkynes
An alkane is a hydrocarbon that has only single bonds. Alkanes that do not contain rings have the formula CnH2n + 2. An alkane in the shape of a ring is called a cycloalkane. Cycloalkanes have the formula CnH2n. An alkene is a compound that has at least one double bond. Straight-chain alkenes with one double bond have the same formula as cycloalkanes, CnH2n. A double bond involves two pairs of electrons. In a double bond, one pair of electrons forms a single bond and the other pair forms an additional, weaker bond. The electrons in the additional, weaker bond react faster than the electrons in the single bond. Thus, carbon-carbon double bonds are more reactive than carbon-carbon single bonds. When an alkene reacts, the reaction almost always occurs at the site of the double bond.
A functional group is a reactive group of bonded atoms that appears in all the members of a chemical family. Each functional group reacts in a characteristic way. Thus, functional groups help to determine the physical and chemical properties of compounds. For example, the reactive double bond is the functional group for an alkene. In this course, you will encounter many different functional groups. An alkyne is a compound that has at least one triple bond. A straightchain alkyne with one triple bond has the formula CnH2n – 2. Triple bonds are even more reactive than double bonds. The functional group for an alkyne is the triple bond.
Figure 1.7 gives examples of an alkane, a cycloalkane, an alkene, and an alkyne.
General Rules for Naming Organic Compounds
The International Union of Pure and Applied Chemistry (IUPAC) has set standard rules for naming organic compounds. The systematic (or IUPAC) names of alkanes and most other organic compounds follow the same pattern, shown below.
The Root: How Long Is the Main Chain?
The root of a compound’s name indicates the number of carbon atoms in the main (parent) chain or ring. Table 1.2 gives the roots for hydrocarbon chains that are up to ten carbons long. To determine which root to use, count the carbons in the main chain, or main ring, of the compound. If the compound is an alkene or alkyne, the main chain or ring must include the multiple bond.
The Suffix: What Family Does the Compound Belong To?
The suffix indicates the type of compound, according to the functional groups present. (See Table 1.4 on page 22.) As you progress through this chapter, you will learn the suffixes for different chemical families. In your previous chemistry course, you learned the suffixes -ane for alkanes, -ene for alkenes, and -yne for alkynes. Thus, an alkane composed of six carbon atoms in a chain is called hexane. An alkene with three carbons is called propene.
The Prefix: What Is Attached to the Main Chain?
The prefix indicates the name and location of each branch and functional group on the main carbon chain. Most organic compounds have branches, called alkyl groups, attached to the main chain. An alkyl group is obtained
by removing one hydrogen atom from an alkane. To name an alkyl group, change the -ane suffix to -yl. For example, CH3 is the alkyl group that is derived from methane, CH4. It is called the methyl group, taken from the root meth-. Table 1.3 gives the names of the most common alkyl groups.
Read the steps below to review how to name hydrocarbons. Then examine the two Sample Problems that follow.
How to Name Hydrocarbons
| Step_1 | Find the root: Identify the longest chain or ring in the hydrocarbon. If the hydrocarbon is an alkene or an alkyne, make sure that you include any multiple bonds in the main chain. Remember that the chain does not have to be in a straight line. Count the number of carbon atoms in the main chain to obtain the root. If it is a cyclic compound, add the prefix -cyclo- before the root. |
| Step_2 | Find the suffix: If the hydrocarbon is an alkane, use the suffix -ane. Use -ene if the hydrocarbon is an alkene. Use -yne if the hydrocarbon is an alkyne. If more than one double or triple bond is present, use the prefix di- (2) or tri- (3) before the suffix to indicate the number of multiple bonds. |
| Step_3 | Give a position number to every carbon atom in the main chain. Start from the end that gives you the lowest possible position number for the double or triple bond, if there is one. If there is no double or triple bond, number the compound so that the branches have the lowest possible position numbers. |
Step_4 | Find the prefix: Name each branch as an alkyl group, and give it a position number. If more than one branch is present, write the names of the branches in alphabetical order. Put the position number of any double or triple bonds after the position numbers and names of the branches, just before the root. This is the prefix. Note: Use the carbon atom with the lowest position number to give the location of a double or triple bond. |
| Step 5 | Put the name together: prefix + root + suffix. |
Aromatic Compounds
If you completed the Concept Check activity on page 12, you drew a possible structure for benzene. For many years, scientists could not
determine the structure of benzene. From its molecular formula, C6H6,
scientists reasoned that it should contain two double bonds and one triple
bond, or even two triple bonds. Benzene, however, does not undergo the
same reactions as other compounds with double or triple bonds.
We know today that benzene is a cyclic compound with the equivalent
of three double bonds and three single bonds, as shown in Figure 1.9(A).
However, the electrons that form the double bonds in benzene are spread
out and shared over the whole molecule. Thus, benzene actually has six
identical bonds, each one half-way between a single and a double bond.
These bonds are much more stable than ordinary double bonds and do
not react in the same way. Figure 1.9(B) shows a more accurate way to
represent the bonding in benzene. Molecules with this type of special
electron sharing are called aromatic compounds. As mentioned earlier,
benzene is the simplest aromatic compound.
Figure 1.10 illustrates some common aromatic compounds. To name
an aromatic compound, follow the steps below. Figure 1.11 gives an
Naming an Aromatic Hydrocarbon
Step 1 Number the carbons in the benzene ring. If more than one type of branch is attached to the ring, start numbering at the carbon with the highest priority (or most complex) group. (See the Problem Tip.)
Step 2 Name any branches that are attached to the benzene ring. Give these branches position numbers. If only one branch is attached to a benzene ring, you do not need to include a position number.
Step 3 Place the branch numbers and names as a prefix before the root, benzene.
Chemists do not always use position numbers to describe the branches CHEM that are attached to a benzene ring. When a benzene ring has only two branches, the prefixes ortho-, meta-, and para- are sometimes used instead
of numbers.
Challenges at work
Listen to four people talking about different business challenges to practise and improve your listening skills.
Do the preparation task first. Then listen to the audio and do the exercises.
Preparation
See MoreListening C1: Challenges at work – preparation
Match the words with the definitions.
Transcript
See MoreA
I’ll never forget the first time I met our new contact from Retrolink, one of our top five customers. I had travelled to their office to meet him in person, talk about the history of our companies together and define a shared vision for future co-operation. I wanted to focus on building a good relationship with him which would be a good foundation for working together.
So, when we met I wanted to make a good impression and also show him respect. I greeted him with a handshake and addressed him by his surname. When I put my hand out I realised he had been moving in to give me a hug, so we did an awkward mixture of the two! Also, when I greeted him by his surname, he responded politely, though he used my first name. It was all a little uncomfortable.
In this situation, I guess I made assumptions about the level of formality he expected. Even though he had never met me before, he knew our two companies had been doing business with each other for years and he wanted to build on that history by being less formal with me from the beginning. The real learning here for me was that I shouldn’t assume we all have the same ideas about meeting people for the first time.
B
I’d never worked with a virtual team on an international project before. It was quite exciting, though also challenging to work with people from different countries, many of whom I would never actually get to meet in person. There were a lot of things that were different about working in the same office and it was quite challenging at the beginning.
For example, there were different time zones, different IT systems and even different local regulations, which impacted on what each person was allowed to, or able to, do for the project.
There were also interpersonal differences, such as different ways of working, approaches to deadlines and when to reply to emails. And we shouldn’t forget the fact that English was the project language and that most people on the team had to work in a language which wasn’t their first. A positive of this was that it meant that everyone, including the native English speakers, had to make the effort to communicate clearly and clarify their own and everyone else’s understanding.
To help us all get on the same page, we defined the communication norms and the rules the team would follow, as well as the meeting dates and deadlines. We laid this all out in a document called a team or project charter. This was really useful and in the end the project was a great success. I’m looking forward to working on my next international project.
C
I went through a bit of a rough patch last year at work. I was already committed to too much and then we lost a team member through restructuring and I quickly became overloaded. This led to me doing too much overtime and feeling very stressed. My boss was really helpful and she pushed back against the unrealistic targets that had been set for our department. She also introduced me to the smart approach to goal setting. It’s an acronym, S-M-A-R-T. You use it to create goals that are Specific, Measurable, Achievable, Relevant and Timely.
I used this approach to deal with all of the things that were overloading me. It helped me to prioritise some tasks, to do some later and to drop some completely.
I often use this approach now and feel much more in control of my time and workload.
D
Have you ever missed a flight or had one cancelled? I did. It happened to me last week. My flight home from a business trip was the last one of the day and we were told it was going to be delayed. That’s always a risk at the end of each day. The ground crew kept extending the delay until eventually they cancelled the flight completely. They then told us to go back through the airport to the departures area to talk to their agent who would organise hotels for everyone and rebook us on the following day’s flight.
As soon as they made the announcement about the cancellation, I knew I had to think quickly as it would not be likely that the flight would have capacity to take everyone from my cancelled flight. I hurried back through the airport and was one of the first to make it to the desk. That turned out to be a good idea, as there were only nine seats available on the flight the next morning. Everyone else had to fly to a different airport and then continue back to our destination airport in coaches. Lucky me! Right?
Task 1
See MoreTask 2
See MoreJoining a gym
Listen to the conversation about joining a gym to practise and improve your listening skills.
Do the preparation task first. Then listen to the audio and do the exercises.
Preparation
See MoreListening B2: Joining a gym – preparation
Match the words in capital letters with the definitions.
Transcript
See MoreReceptionist: Hello, welcome to Shake it Off Fitness. How can I help you?
Customer: Yes, thanks. I’ve been meaning to come in and find some information about here for some time. I was wondering if you could help me?
Receptionist: Sure, of course. What would you like to know?
Customer: I saw on the sign outside that you have the best budget membership in the city. Is that right?
Receptionist: You’ve got that right, yes. If you find a better rate, then we’ll match it. Also, we don’t have any sign-up fees or cancellation fees.
Customer: Sorry, what was that last bit?
Receptionist: Cancellation fees. Some gyms charge you money if you leave before a minimum number of months or something like that. We don’t do that.
Customer: That’s useful to know. Ummm, what times do you have?
Receptionist: We open at 5 a.m. and close at midnight.
Customer: Not 24 hours then, I see.
Receptionist: No. Almost, though! We tried doing the 24-hour thing, but there really weren’t a lot of people who wanted to do exercise at 2 in the morning.
Customer: Makes sense. Another question: do you have trainers? Like, will you do a personalised evaluation of me?
Receptionist: Sure we do. As we say on our brochure, we can show you what to do, how to do it and why you’re doing it. Have you worked with a personal trainer before?
Customer: Err, no. No, I haven’t. No.
Receptionist: So, it’s very easy. For your first visits we’ll assign you one of our expert personal trainers. He or she will design an exercise plan that’s just right for you and show you exactly what to do. We even include nutrition advice in the plan.
Customer: Let me get this straight. All of this is included in the price?
Receptionist: Yes, it is. If you want to continue with your trainer after the first few classes, we can talk about that. It does cost extra though. OK?
Customer: Yeah. Sounds … great.
Receptionist: Do you have any other questions?
Customer: Hmmm, yeah. Um, OK. Why should I choose here instead of any of the other gyms in the city?
Receptionist: What, you mean apart from our competitive rates and personalised attention?
Customer: Yeah, I guess.
Receptionist: Listen, here’s a free day pass. Why don’t you drop by and see for yourself? That way you can talk to other members, see the facilities and our staff.
Customer: OK, I’ll do that. Thanks very much!
Task 1
See MoreTask 2
See MoreChatting about a series
Listen to two friends chatting about a television series to practise and improve your listening skills.
Do the preparation task first. Then listen to the audio and do the exercises.
Preparation
See moreListening B1: Chatting about a series
Match the words with the explanations.
Transcript
See moreMan: So, did you see the final episode last night?
Woman: Of course! As if I would miss that!
Man: And?
Woman: It was by far the best episode, definitely.
Man: Yeah, totally. The dragon at the end and all the special effects in that final scene were amazing.
Woman: I loved that part! But I still don’t think this series was as good as the others.
Man: Really? But you said you wouldn’t want to miss it?
Woman: I know, but still … it’s a lot more predictable than it used to be.
Man: But that’s because the story has been told so well that all the characters are reaching their destiny now.
Woman: I don’t know … before, anyone could die at any time so it was exciting! But, this series, no key characters died and we all knew they wouldn’t. When Jaime fell in the water after the dragon attacked him, we knew he would make it somehow. And … oh, surprise, Bronn jumps in and saves him.
Man: They did kill some characters in this series, though. What was that one’s name …?
Woman: Exactly, see! You can’t even remember who died!
Man: Hmm. The only thing I didn’t like about this series was that it was shorter than the others. Seven episodes instead of ten. Maybe they spent all their money on that dragon!
Woman: Right! I don’t understand why they did it, as all the fans would happily watch ten episodes.
Man: I reckon that now they’re not using the story in the books any more, they don’t have as many ideas.
Woman: That would explain why they’re not being as brave with the story too. It’s more like a Hollywood film than a TV show now.
Man: So, is Tyrion still your favourite character?
Woman: You know … surprisingly, I think my favourite might be Cersei.
Man: What?!
Woman: She’s so interesting! All the awful things she’s done and the way she’s just aiming for revenge, even though it won’t make her happy. She still surprises us because we’re expecting her to have a happy ending where she sees her mistakes and becomes a good person. But she never does, no matter what it costs her. She’s unpredictable because we just can’t believe anyone would be like her.
Man: I guess. You don’t have to like a character for them to be your favourite!
Task 1
See moreListening B1: Chatting about a series – 1
Match the characters with what the speakers say about them.
Task 2
See moreLeaving a message
Do the preparation task first. Then listen to the audio and do the exercises.
Listen to someone leaving a message to practise and improve your listening skills.
Preparation
See MoreTranscript
See MoreJane: Hello, this is the sales department. Jane Solomon speaking.
Peter: Hello, is Maria Fernandez there, please?
Jane: No, I’m sorry. She’s not in the office at the moment. She’s on her lunch break.
Peter: Oh. Could you take a message, please? Could you tell her that Peter Griffin called?
Jane: Sure. Could you give me your number?
Peter: It’s 0-7-4-6-0 double 9-0-1 double 8.
Jane: Thanks. That’s 0-7-4-6-0 2-9-0-1-2-8.
Peter: Sorry, no. It’s 0-7-4-6-0 9-9-0-1-8-8.
Jane: 9-9-0-1-8-8.
Peter: Yes. Please ask her to call me back, and tell her it’s about the PXO project. I need the new project figures.
Jane: The PXO project. Right. I’ll give her your message when she comes back to the office.
Peter: Thank you. In case I’m in a meeting when she calls back, can I give you my email address too?
Jane: Of course.
Peter: Great. It’s P-E-T-E-R dot G-R-I-F-F-I-N at F-R-E-S-H dot com.
Jane: Can I read that back to you?
Peter: Sure.
Jane: That’s P-E-T-E-R dot G-R-I-F-F-I-N at F-R-E-S-H dot com.
Peter: Yes, that’s it.
Jane: OK. I’ll tell her you called.
Peter: Thank you. Goodbye.
Jane: Goodbye.
Task 1
See MoreTask 2
See MoreMeeting other students
Listen to a group of new students meeting for the first time to practise and improve your listening skills.
Do the preparation task first. Then listen to the audio and do the exercises.
Preparation
See MoreListening A1: Meeting other students – preparation
Transcript
See MoreTeacher: So, now you’ve got the important information, it’s time to meet each other. Everyone turn to the people next to you and introduce yourselves.
Cara: So, hi. I’m Cara. And you are?
Robert: Robert.
Selim: Selim.
Cara: Nice to meet you!
Robert and Selim: You too.
Robert: Nice accent. Where are you from?
Cara: I’m from Glasgow.
Robert: Oh, really? My mum’s from near Glasgow, so I’m half Scottish.
Cara: Cool. Do you live in Scotland?
Robert: No, we live in England, near Manchester. My dad’s from there. What about you, Selim?
Selim: I’m from Leeds originally but I grew up near London.
Cara: Are you both doing history on its own?
Selim: No. I’m doing history and German.
Robert: I’m doing it with maths, actually.
Selim: History and maths. That’s different!
Robert: Yeah, I couldn’t decide between arts and sciences. Maths doesn’t help with remembering dates, though! And you?
Cara: I’m doing history and French.
Selim: I wanted to do French but German was easier, so I took that.
Cara: German is so hard!
Task 1
See MoreListening A1: Meeting other students – 1
Put the phrases in the correct group.
Task 2
See MoreListening A1: Meeting other students – 2
Biological Macromolecules and Their Subunits
The atoms of four elements make up roughly 99 percent of the mass of most cells: hydrogen, nitrogen, carbon, and oxygen. With only a few exceptions, molecules that contain carbon atoms are called organic compounds. There are millions of different organic compounds. Nearly all organic compounds contain hydrogen as well as carbon, and most of these also include oxygen. Pure carbon and carbon compounds that lack hydrogen — such as carbon dioxide and calcium carbonate — are considered inorganic. Inorganic compounds are, nevertheless, integral components of living systems. See Figure 1.9. For example, water — an inorganic compound — provides a medium in which various substances may be dissolved and transported within and between cells.
Figure 1.9 In what ways do living and non-living systems,
and organic and inorganic compounds interact?
The Central Atom: Carbon
The diversity of life relies greatly upon the versatility of carbon. Recall that a carbon atom in its most stable state has two occupied energy levels, the second of which contains four valence electrons. This means that, in covalent molecules, a carbon atom can form bonds with as many as four other atoms. In biological systems, these atoms are mainly hydrogen, oxygen, nitrogen, phosphorus, sulfur, and — importantly — carbon itself. Carbon’s ability to bond covalently with other carbon atoms enables carbon to form a variety of geometrical structures, including straight chains, branched chains, and rings. Figure 1.10 shows the shapes of several simple organic molecules that contain only carbon and hydrogen atoms. These molecules, called hydrocarbons, comprise the fossil fuels that serve as the main fuel source for much of the world’s industrial activities. Hydrocarbons are themselves not components of living systems. However, substantial portions of many biological molecules consist of bonded chains of carbon and hydrogen.
Molecular Isomers
Because carbon can form so many compounds with so many elements, it is common to encounter several organic compounds with the same molecular formula but different structures. Such compounds are known as isomers. For example, two isomers of glucose, a six-carbon sugar, are fructose and galactose. Glucose, fructose, and galactose all have the same molecular formula (C6H12O6). However, they differ in their molecular structures, as shown in Figure 1.11.
There are two main types of isomers. Structural isomers are two or more compounds with the same atoms bonded differently. Glucose and fructose, for example, are structural isomers. Notice that a glucose molecule contains a ring of five carbon atoms and an oxygen atom, whereas a fructose molecule contains a ring of four carbon atoms and an oxygen atom. Because their structures are different, glucose and fructose have different properties, and cells metabolize them differently.
Stereoisomers are two or more compounds with their atoms bonded in the same way, but with atoms arranged differently in space. Stereoisomers may be geometrical or optical. Geometrical isomers can have very different physical properties (such as different melting points), but they tend to have the same chemical properties. Glucose and galactose are examples of geometrical isomers.
Optical isomers, shown in Figure 1.12, are nonsuperimposable mirror images of each other. They usually have similar chemical and physical properties, but enzymes or proteins on the cell membrane can distinguish between them. Usually, one optical isomer is biologically active and the other biologically inactive. In some cases however, this is not always true. For example, sometimes one optical isomer of a drug is not as effective as the other or can even cause complications. In the early 1960s, many pregnant women were prescribed a drug called thalidomide for morning sickness.
Thalidomide is a mixture of two optical isomers; one produced the desired effect, but the other caused major birth defects. As the thalidomide example demonstrates, organisms can be very sensitive to minute variations in molecular geometry.
The Functional Groups
Chemical reactions involve breaking or forming chemical bonds. These processes can transform simple molecules such as glucose into complex molecules such as starch or cellulose. Many of these complex molecules contain groups of atoms with characteristic chemical properties. These groups of atoms, known as functional groups, include hydroxyl, carbonyl, carboxyl, amino, sulfhydryl, and phosphate groups, as shown in Figure 1.13 Many compounds have more than one functional group in their structure.
These functional groups are hydrophilic. Except for the phosphate group, they are polar and so they increase the solubility in water of the organic molecules to which they are attached. Each functional group also has capabilities to change the chemical properties of the organic molecules to which it bonds. For example, if a hydrogen atom in
ethane is replaced by a sulfhydryl group, the result is ethanethiol, also known as ethyl mercaptan. While ethanethiol in small amounts stabilizes protein structures, it is also a dangerous neurotoxin and respiratory toxin. Each functional group has a specific role in cell metabolism. Phosphates are essential to the metabolic processes of photosynthesis and cellular respiration. For example, the transfer of a phosphate group from ATP (adenosine triphosphate) begins the very important process of glycolysis — the first step in cellular respiration. You will discover more about this process in Chapter 3.
While amino and phosphate groups contribute to energy transactions in the cell, the sulfhydryl (–SH) group is essential to protein stabilization. Amino acids with –SH groups form bonds called disulfide bridges (S–S bonds) that help protein molecules to take on and maintain a specific shape.
Monomers and Macromolecules
As you know, atoms can join together — bond — to form small compounds called molecules. Similarly, molecules can join together to form large structures called macromolecules. The small, molecular subunits that make up macromolecules are called monomers. The macromolecules themselves are built up of long chains of monomers. These chains are called polymers.
Table 1.2 lists the main types of macromolecules and their monomer subunits. Figure 1.14 depicts the subunits that comprise carbohydrates, selected lipids, proteins, and nucleic acids. Chemical reactions in cells synthesize macromolecules from these subunits, and break the molecules apart to release their subunits. Refer to Figure 1.14 often as you examine these chemical reactions in the final
section of this chapter.
Bonding and the Shape of Organic Molecules
Early scientists defined organic compounds as compounds that originate from living things. In 1828, however, the German chemist Friedrich Wohler (1800–1882) made an organic compound called urea, CO(NH2)2, out of an
inorganic compound called ammonium cyanate, NH4CN. Urea is found in the urine of mammals. This was the first time in history that a compound normally made only by living things was made from a non-living substance. Since Wohler had discovered that organic compounds can be made without the involvement of a life process, a new definition was required.
Organic compounds are now defined as compounds that are based on carbon. They usually contain carbon-carbon and carbon-hydrogen bonds.
The Carbon Atom
There are several million organic compounds, but only about a quarter of a million inorganic compounds (compounds that are not based on carbon). Why are there so many organic compounds? The answer lies in the bonding properties of carbon.
As shown in Figure 1.1, each carbon atom usually forms a total of four covalent bonds. Thus, a carbon atom can connect to as many as four other atoms. Carbon can bond to many other types of atoms, including hydrogen, oxygen, and nitrogen.
In addition, carbon atoms can form strong single, double, or triple bonds with other carbon atoms. In a single carbon-carbon bond, one pair of electrons is shared between two carbon atoms. In a double bond, two pairs of electrons are shared between two atoms. In a triple bond, three pairs of electrons are shared between two atoms. Molecules that contain only single carbon-carbon bonds are saturated. In other words, all carbon atoms are bonded to the maximum number of other atoms: four. No more bonding can occur. Molecules that contain double or triple carbon-carbon bonds are unsaturated. The carbon atoms on either side of the double or triple bond are bonded to less than four
atoms each. There is potential for more atoms to bond to each of these carbon atoms.
Carbon’s unique bonding properties allow the formation of a variety of structures, including chains and rings of many shapes and sizes. Figure 1.2 on the next page illustrates some of the many shapes that can be formed from a backbone of carbon atoms. This figure includes examples of three types of structural diagrams that are used to depict organic molecules. (The Concepts and Skills Review contains a further review of these types of structural diagrams.)
(B) This condensed structural diagram shows only carbon-carbon bonds.
(C) This linestructural diagram uses lines to depict carbon-carbon bonds.
Carbon compounds in which carbon forms only single bonds have a different shape than compounds in which carbon forms double or triple bonds. In the following ExpressLab, you will see how each type of bond affects the shape of a molecule.
the shape of a molecule depends on the type of bond. Table 1.1 describes some shapes that you must know for your study of organic chemistry. In Unit 2, you will learn more about why different shapes and angles form around an atom.
Three-Dimensional Structural Diagrams
Two-dimensional structural diagrams of organic compounds, such as condensed structural diagrams and line structural diagrams, work well for flat molecules. As shown in the table above, however, molecules containing single-bonded carbon atoms are not flat.
You can use a three-dimensional structural diagram to draw the tetrahedral shape around a single-bonded carbon atom. In a three-dimensional diagram, wedges are used to give the impression that an atom or group is coming forward, out of the page. Dashed or dotted lines are used to show that an atom or group is receding, or being pushed back into the page. In Figure 1.3, the Cl atom is coming forward, and the Br atom is behind. The two H atoms are flat against the surface of the page.
Molecular Shape and Polarity
The three-dimensional shape of a molecule is particularly important when the molecule contains polar covalent bonds. As you may recall from your previous chemistry course, a polar covalent bond is a covalent bond between two atoms with different electronegativities.
Electronegativity is a measure of how strongly an atom attracts electrons in a chemical bond. The electrons in a polar covalent bond are attracted more strongly to the atom with the higher electronegativity. This atom has a partial negative charge, while the other atom has a partial positive charge. Thus, every polar bond has a bond dipole: a partial negative charge and a partial positive charge, separated by the length of the bond.
Figure 1.4 illustrates the polarity of a double carbon-oxygen bond. Oxygen has a higher electronegativity than carbon. Therefore, the oxygen atom in a carbon-oxygen bond has a partial negative charge, and the carbon atom
has a partial positive charge.
Other examples of polar covalent bonds include CO, OH, and NH. Carbon and hydrogen attract electrons to almost the same degree. Therefore, when carbon is bonded to another carbon atom or to a hydrogen atom, the bond is not usually considered to be polar. For example, CC bonds are considered to be non-polar.
Predicting Molecular Polarity
A molecule is considered to be polar, or to have a molecular polarity, when the molecule has an overall imbalance of charge. That is, the molecule has a region with a partial positive charge, and a region with a partial negative charge. Surprisingly, not all molecules with polar bonds are polar molecules. For example, a carbon dioxide molecule has two
polar CO bonds, but it is not a polar molecule. On the other hand, a water molecule has two polar OH bonds, and it is a polar molecule.
How do you predict whether or not a molecule that contains polar bonds has an overall molecular polarity? To determine molecular polarity, you must consider the shape of the molecule and the bond dipoles within the
molecule.
If equal bond dipoles act in opposite directions in three-dimensional space, they counteract each other. A molecule with identical polar bonds that point in opposite directions is not polar. Figure 1.5 shows two examples, carbon dioxide and carbon tetrachloride. Carbon dioxide, CO2, has two polar CO bonds acting in opposite directions, so the molecule
is non-polar. Carbon tetrachloride, CCl4, has four polar CCl bonds in a tetrahedral shape. You can prove mathematically that four identical dipoles, pointing toward the vertices of a tetrahedron, counteract each other exactly. (Note that this mathematical proof only applies if all four bonds are identical.) Therefore, carbon tetrachloride is also non-polar.
If the bond dipoles in a molecule do not counteract each other exactly, the molecule is polar. Two examples are water, H2O, and chloroform, CHCl3, shown in Figure 1.6. Although each molecule has polar bonds, the bond dipoles do not act in exactly opposite directions. The bond dipoles do not counteract each other, so these two molecules are polar.
The steps below summarize how to predict whether or not a molecule is polar. The Sample Problem that follows gives three examples.
Note: For the purpose of predicting molecular polarity, you can assume that CH bonds are non-polar. In fact, they have a very low polarity.
Step 1 Does the molecule have polar bonds? If your answer is no, see below. If your answer is yes, go to step 2.
If a molecule has no polar bonds, it is non–polar. Examples: CH3CH2CH3, CH2CH2
Step 2 Is there more than one polar bond? If your answer is no, see below. If your answer is yes, go to step 3.
If a molecule contains only one polar bond, it is polar. Examples: CH3Cl, CH3CH2CH2Cl
Step 3 Do the bond dipoles act in opposite directions and counteract each other? Use your knowledge of three-dimensional molecular shapes to help you answer this question. If in doubt, use a molecular model to help you visualize the shape of the molecule.
If a molecule contains bond dipoles that do not counteract each other, the molecule is polar. Examples: H2O, CHCl3
If the molecule contains dipoles that counteract each other, the molecule is non–polar. Examples: CO2, CCl4
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