CHEM 191

MODULE 7

ORGANIC CHEMISTRY 1

Learning Objectives.

By the end of this module you should be able to:

• Write names and structural formulae for simple alkanes, alkenes and alkynes including constitutional isomers

• Identify and name stereoisomers of alkenes (cis and trans)

• Identify combustion, oxidation, substitution, elimination and addition reactions of organic molecules by comparing the structural formulae of reactants and products.

• Write equations using structural formulae for reactions of alkanes, alkenes and alkynes

• Recognise the monomers and polymers in addition polymerisation reactions

Reference: ESA Chapters 12 and 13

INTRODUCTION

The term ‘organic’, in common usage refers to the products of agriculture and farming that are produced according to particular methods of fertilisation and pest control.

In chemistry, the term organic refers to the study of compounds of carbon (excluding CO, CO2  and the carbonates). Organic chemistry was once referred to as the chemistry of ‘living things’ and it was thought that organic compounds could not be synthesised in a laboratory because they all contained a ‘vital force’. The synthesis of urea by Fredrick Wholer in 1828 proved that organic compounds could be manufactured and there are now millions of different organic compounds known, with new ones discovered each year. Organic compounds are found in living organisms, fossil fuels such as coal and petroleum, and many common products such as soap, plastics, paper, cosmetics and medicines.

Carbon is unique among the elements in its ability to form chains, branched chains and rings of carbon atoms of an almost unlimited size and variety. It is able to form 4 stable bonds with itself and other atoms, predominately hydrogen but also oxygen, sulfur, nitrogen, phosphorus and the halogens.

HYDROCARBONS

The simplest organic compounds contain only carbon and hydrogen and are called hydrocarbons. There are 3 groups of hydrocarbons:

•     Alkanes –molecules that have only single bonds between the carbon atoms. They are said to be saturated because they have the maximum number of hydrogen atoms possible for the given number of carbon atoms.

•     Alkenes –molecules with one or more double bond between the carbon atoms. They are said to be unsaturated because it is possible to add hydrogen atoms to them

•     Alkynes –molecules with one or more triple bonds between the carbon atoms. This class of compounds is also unsaturated.

ALKANES

Alkanes are the simplest organic compounds and can be regarded as the “parents” of all other organic compounds. Since a carbon atom must form. four covalent bonds to either itself or other atoms, the simplest hydrocarbon will therefore have the formula CH4.

CH4 is known as methane and methane molecules have the following Lewis diagram and structure:

Methane is a colourless, odourless gas that is the main component of “Natural Gas” which we burn for fuel. It is a “greenhouse gas’ which, along with carbon dioxide, makes a significant contribution to global warming.

Methane is the first of the alkane family of hydrocarbons. When a second carbon is introduced to the molecule we need 6 hydrogen atoms to satisfy the bonding requirements of the carbon atoms. The compound has the molecular formula C2H6 and is called ethane.

In organic chemistry it is usual to use a structural formula rather than the molecular formula. However, it is convenient to condense the structural formula so that all the bonds to hydrogen are not shown, while still retaining the carbon to carbon framework. This gives the condensed structural formula for ethane as CH3CH3.

When a third carbon atom is introduced, the compound is known as propane.

Molecular formula: C3H8                     Condensed structural formula: CH3CH2CH3

Structural formula: 

The four carbon alkane, C4H10, is known as butane. The five carbon alkane, C5H12, is pentane. If we keep adding carbon atoms to the chain we can develop a series of molecules that differ by a –CH2-   group.

Butane:  CH3CH2CH2CH3, Pentane: CH3CH2CH2CH2CH3.

The names and formulae of the first ten alkanes are given below

Table 7.1        The First Ten Unbranched Hydrocarbons

All the alkanes can be represented using the general formula CnH2n+2  where n represents some integer.

As the length of the hydrocarbon chain increases it is sometimes more convenient to summarise the number of CH2 groups; pentane could be written as CH3(CH2)3CH3  and decane as CH3(CH2)8CH3. The naming of these straight chain alkanes is relatively simple. Apart from the first four members of the series, the name is either a Greek or Latin prefix indicating the number of carbon atoms in these molecules and this is followed by the ending -ane.

(Hint: To remember the order of the names for the first four hydrocarbons you could use the mnemonic: Must Eat Peanut Butter)

Focussing Questions 1

1.    What is a hydrocarbon?

2.    What is the difference between a saturated and an unsaturated  molecule?

3.    What are the similarities and differences between alkanes, alkenes and alkynes?

4.    What are the names of the first 4 alkanes?

5.    What is the general formula for an alkane molecule (ratio of C to H atoms)?

6.    What is the shape of the bonding around each C atom in an alkane? Give a reason for your answer.

Straight Chains, Branched Chains, and Rings in Alkanes

The molecules in Table 7.1 are all examples of so-called straight chain alkanes – each carbon atom (except for those at the two ends of the chain) is connected to two other carbon atoms. In branched- chain alkane molecules some of the carbon atoms are attached to more than two other carbon atoms.

For example,

A count of the atoms will show that branched-chain alkanes still have the general formula CnH2n+2 . A branched chain molecule is considered to have a ‘long’ chain to which ‘side’ chains are attached. Side  chains are are known as alkyl groups. An alkyl group is formed by removing one hydrogen atom from an alkane and is named by replacing the –ane ending with  -yl. Thus CH3- is methyl- from methane, CH3CH2- is ethyl- from ethane, CH3CH2CH2- is propyl, etc. The significance of “group” is that it refers to a cluster of atoms that are a part of a molecule, where the cluster cannot exist on its own.

Table 7.2           Names of some side chains

In cycloalkanes, three or more carbon atoms are linked together in a ring. Ring sizes can be up to 30 or more carbon atoms.

For example:

Structurally, the difference between a straight chain alkane and a cycloalkane is the absence of one hydrogen atom from each end of the straight chain. Consequently, the general formula for any cycloalkane is CnH2n.

When drawing structural diagrams for complex structures, including cycloalkanes, the C and H atoms are not explicitly written. Instead, a line diagram is used, in which the C-C bonds are shown as lines with the end of each line being a CH, CH2  or CH3  as required for the structure to be neutral.

For example:

ISOMERISM

In inorganic chemistry, with very few exceptions, the molecular formula of a compound uniquely identifies that compound. For organic compounds, the molecular formula is rarely unique. Different compounds with the same molecular formula but a different arrangement of atoms are said to be isomers of each other. Structural or constitutional isomers arise when the same atoms can be connected together in different ways. For all hydrocarbons larger than propane, there is more than one way in which the carbon atoms can be interconnected.

Butane (C4H10) has two isomers:

The second of these two isomers is an example of a branched chain isomer while the first is known as a straight chain isomer.

Pentane (C5H12) has three isomers:

Isomers have different boiling points, freezing points, and densities. The existence of isomers is one of  the reasons for the vast number of organic compounds. As the number of carbon atoms is increased, the number of possible isomers increases rapidly – C8H18  has 18 possible isomers, C20H42 has 366319 isomers and C40H82  has an estimated 6.25 × 1013  possible isomers.

Exercise 7.1

(a)        Draw full structural formulae for the following alkane molecules:

(i)         Propane

(ii)        A straight chain alkane with 5 carbons

(iii)       The alkane C7H16

(b)        Draw condensed structural formulae for each of the molecules in (a)

(c)        Write the molecular formula for alkane molecules with the following number of carbon atoms:

(i)  12                 (ii)  27                (iii)        8

(d)        Draw the condensed structural formulae for 5 different arrangements (isomers) of the formula C6H14

Naming Branched-Chain Alkanes

An international set of rules has been developed by the International Union of Pure and Applied Chemistry (IUPAC) for naming organic compounds. The name will indicate the number of carbon atoms in the molecule, the way these are arranged and, when atoms other than hydrogen and carbon are involved,where these are attached and in what order.

The IUPAC name for a straight chain alkane consists of:

•    a prefix which shows the number of carbons

•    the suffix –ane which shows the hydrocarbon belongs to the alkane family (see Table 7.1).

For branched-chain isomers we need to show:

•    The parent name which is the longest chain of carbon atoms

•    The substituents or branches (sometimes known as side chains) and their position on the parent chain.

To name a molecule:

1. Identify the longest continuous chain of carbon atoms (the parent chain).

2. Number each carbon of the parent chain, starting from whichever end of the chain gives the lowest numbers to carbon atoms substituted with the branches or side chains.

3. Identify the alkyl group(s)which forms the side chain(s)

4. Prefix the name of the side chain(s) to the name of the parent chain and identify the number(s) of the carbon atom(s) to which it is attached

Example 1:  To name the molecule:

1.  Identify the longest continuous chain of carbon atoms in the structure and let this be the parent chain.

•    There are 6 molecules in the longest chain so the parent chain is hexane.

(Note that for simplicity in this and in the following examples, the molecules have been

represented with the longest chain in a straight line but this is not a requirement, nor will it always be the case.)

2.  Number each carbon of the parent chain, starting from whichever end of the chain gives the lower number to carbon atom with the branches or side chains.

•    Had we numbered from right to left, the side chain carbon atom would have been number 4.

3.  Identify the alkyl group which forms the branch

•    In this case it is a methyl group (as it has only 1 carbon atom)

4.  Prefix the name of the side chain to the name of the parent chain and identify the number of the carbon atom to which it is attached.

•    The correct name is 3-methylhexane.

Note: There are no spaces between the two parts of the name and the location number is separated from the name by a hyphen.

Example 2. What if there is more than one side chain?

To name the molecule:

1.  Parent chain = heptane

2.  Number the parent carbon chain from the end which gives the lowest sum for the carbons on which the side chains are found.

•    In the example above, there are two options – numbering from the left, the side chains would be on carbons 3 and 6 (3 + 6 = 9). Numbering from the right the side chains would be on carbons 2 and 5 (2 + 5 =7).  Numbering from the right gives the lowest sum.

3.  Alkyl groups are: methyl on carbon 2 and ethyl (2 carbon chain) on carbon 5.

4.  The alkyl group prefixes areplaced in alphabetic order with hyphens used to separate numbers from letters.

The correct name is 5-ethyl-2-methylheptane.

NOTE – The structure in Example 2 was drawn with the parent chain in a straight line. However, you should always check whether there is a longer chain that includes a group that has been drawn as a side chain.

For example – the way the following molecule is drawn the longest chain goes through the side chain.

Example 3.  What if there are two or more identical side chains?

We use di- for two tri- for three, tetra- for four, penta- for 5 etc.to indicate identical side chains.

To name the molecule:

Name is: 2,5-dimethylheptane.

Note: These scaling prefixes do not affect the alphabetic order triethyl still comes before methyl.

Example 4. What if identical groups are on the same carbon?

Where identical groups are on the same carbon atom, repeat the number of this carbon in the name.

To name the molecule:

Naming Cyclic Alkanes

If there is a ring present, it becomes the parent no matter how long the side-chains might be.  The parent name is prefixed by cyclo, and numbering starts at the lowest alphabetic group and proceeds in the direction which gives the lowest overall set of numbers.

Example: To name the compound:  

Name is: 1,2-dimethylcyclohexane (not 1,6-dimethylcyclohexane or 5,6-dimethylcyclohexane or any other combination).

Summary

• Alkyl groups are listed in alphabetic order, ignoring any scaling prefixes

• If there is any ambiguity, numbers are used to identify the carbon atoms to which alkyl side chains are attached.

• Numbers are separated from numbers by commas, numbers are separated from letters by hyphens.

• There are no spaces between letters