Unit 2: The World of Carbon

(b) Nomenclature and structural formulae

 

Hydrocarbons

The naming of organic compounds has been agreed by chemists internationally. This naming is always acceptable although the shorter traditional names are frequently used for convenience.

The system depends on the length of the carbon chain. The compound will be named after the alkane wih the same number of carbons as the longest carbon chain in the molecule,

e.g. if the longest chain of carbon atoms is three the name of the compound is based on propane.

The position of the functional groups is given by numbering each carbon atom in the chain,

e.g. if the three carbon chain contains an OH group on the middle carbon atom the compound is propanol with alcohol group on carbon 2 or propan-2-ol.

A branched carbon chain is dealt with by giving the branches names as if they were functional groups. A single carbon branch is a methyl group, a 2 carbon group an ethyl and so on.

Take butane as an example. Butane has two isomers

The numbering should always be arranged so that the numbers in the name are as low as possible. Thus a branched chain would be called 2-methylbutane rather than 3-methylbutane.

Where there are two or more functional groups they are quoted in alphabetical order with the appropriate number, e.g. 3-ethyl 2- methylhexane.

If the same functional group occurs more than once in a molecule then the name will have a di, tri or tetra before the functional group. For example in 1,1,1-trichloroethane the numbers show that all of the three Cl atoms are on the same C atom.

 

Alkenes

The functional group in an alkene is the carbon to carbon double bond.

C=C

Properties of heptene

1. Immiscible with water

2. pH = 7

3. Burns to give carbon dioxide and water

C7H14 + 10ŻO2 → 7CO2 + 7H2O

4. Decolourises bromine water rapidly by an addition reaction

eg. for ethene

CH2=CH2 + Br2 → CH2Br-CH2Br

dibromoethane

Alkynes

The functional group in an alkyne is the carbon to carbon triple bond.

C ≡ C

Ethyne (acetylene) can be prepared by reacting calcium carbide with water.

CaC2 + 2H2O → C2H2 + Ca(OH)2

Properties of ethyne

1. Immiscible with water

2. pH = 7

3. Burns to give carbon dioxide and water

C2H2 + 2ŻO2 → 2CO2 + H2O

In air the combustion tends to be incomplete and results in a smokey flame.

4. Decolourises bromine water slowly by an addition reaction

eg. for ethyne

CH≡CH + Br2 → CHBr2-CHBr2

tetrabromoethane

 

Substituted alkanes

Alcohols

An alcohol can be identified from the hydroxyl functional group and the '-ol' name ending.

Functional group -OH (hydroxyl)

The alkanols are a homologous series of alcohols based on the corresponding parent alkanes.

name

structure

methanol

CH3OH

ethanol

CH3CH2OH

propanol

CH3CH2CH2OH

butanol

CH3CH2CH2CH2OH

Properties of ethanol

1. Completely miscible with water

2. pH = 7

3. Burn to give carbon dioxide and water

CH3CH2OH + 3O2 → 2CO2 + 3H2O

4. Does not react with magnesium

 

 

Classification of Alkanols

In naming the alkanols, the position of the hydroxyl group is given from the number of the carbon nearest it.

The isomers of alcohols are numerous but they all fall into one of three groups Primary, Secondary or Tertiary.



Aldehydes and Ketones

Aldehydes (alkanals) and ketones (alkanones) contain the same functional group , the carbonyl group

C = O

Alkanals and alkanones can be identified from the '-al' and '-one' name endings.

Aldehydes have a hydrogen atom joined to the carbonyl group whereas ketones have two carbon atoms joined to the carbonyl group.

 

Aldehydes

Alkanals are a homologous series of aldehydes based on the corresponding parent alkanes.

name

structure

methanal

H - CH = O

ethanal

CH3 - CH = O

propanal

CH3 - CH2 - CH = O

 

 

Ketones

Alkanones are a homologous series of ketones based on the corresponding parent alkanes.


name

structure

propanone

butanone

Naming of alkanones is almost exactly the same as for alkanols. e.g. butanone. For longer chain ketones the position of the carbonyl group must be given.

 

pentan-2-one

Alkanoic Acids

A carboxylic acid can he identified from the carboxyl functional group and the '-oic' name ending.

Funtional group -

Alkanoic acids are a homologous series of carboxylic acids based on the corresponding parent alkane.

 

name

structure

formula

methanoic acid

(formic acid)

HCOOH

ethanoic acid

(acetic acid)

CH3COOH

propanoic acid

CH3CH2COOH

butanoic acid

CH3CH2CH2COOH

Properties of ethanoic acid

1. Completely miscible with water

2. ph < 7 weakly acidic

CH3COOH ⇌ CH3COO- + H+

3. Non flammable

4. React with magnesium to give a salt + water

Mg + 2CH3COOH → Mg(CH3COO)2 + H2

 

 

Esters

Esters are formed by the reaction of alcohols with organic acids.

Consider the reaction of the alkanol - methanol with the alkanoic acid - propanoic acid.

The reaction is reversible - an equilibrium is set up.

The reaction is described as a condensation reaction as water is produced. If this water is removed the equilibrium is shifted to the side of the products. Concentrated sulphuric acid has a high affinity for water and is used to remove it and shift the equilibrium to the right.

 

An ester can be identified from the functional group and the '-oate' ending.

Funtional group -

 

Naming Esters

An ester can be named given the name of the parent alkanol and alkanoic acid or from shortened and full structural formulae.

The parent alkanol gives the start of the name ending in -yl. The parent acid gives the second part of the name ending in -oate. So the ester formed by reacting butanol with ethanoic acid would be butyl ethanoate.

Name the following ester

or shortened structural formula

CH3CH2CH2CH2COOCH2CH2CH3

The alcohol must be CH3CH2CH2 - OH (propanol) and

the acid must be CH3CH2CH2CH2 - COOH (pentanoic acid)

The ester is named propyl pentanoate.

 

 

Aromatic hydrocarbons

These compounds were named because of the distinctive smell of many of them.

Benzene (C2H6) is the simplest member of the class of aromatic hydrocarbons.

Benzene is a cyclic compound whose structure is a six membered ring with a hydrogen atom attached to each carbon atom.

It has a distinctive structural formula

A benzene ring in which one hydrogen atom has been substituted by another group is known as the phenyl group.

The phenyl group has the formula -C6H5

 

Properties of Toluene

Benzene has carcinogenic and liver damaging properties and so is banned from educational establishments. Toluene is used as a substitute while doing experiments. Toluene is the compound methyl benzene.

1. It is immiscible with water

2. pH = 7

3. It burns in oxygen

C6H5CH3 + 9O2 → 7CO2 + 4H2O

As seen from the equation, a lot of oxygen is required. In air the combustion tends to be incomplete and results in a smokey flame.

4. Very slow reaction with bromine water - it resists addition reactions.

Since toluene doesn't react with bromine it cannot contain any C=C double bonds.

 

Benzene has been analysed with X-rays and it was discovered that

- the bond angles are all 120o.

- the bond lengths are all equal.

- the bond energy is 510 kJmol-1.

This is somewhere between C-C bonds (337 kJmol-1) and C=C bonds (607 kJmol-1).

Each carbon atom in benzene forms three normal covalent bonds. This accounts for the carbon to carbon bonds and the carbon to hydrogen bonds. One bonding electron is not involved. This leaves a total of six electrons in benzene which are not involved in normal bonding. These electrons occupy space above and below the plane of the molecule and are said to be "delocalised".