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HOMOLOGOUS SERIES,

FORMULAE & IUPAC Keane A. Campbell

MSc; BSc; ASc

September 10, 2013

HOMOLOGOUS SERIES

This is a series of organic compounds with a similar general formula, possessing similar chemical properties due to the presence of the same functional group, and shows a gradation in physical properties as a result of increase in molecular size and mass.

For example, ethane has a higher boiling point than methane since it is more van der Waals forces (intermolecular forces) with neighboring molecules.

This is due to the increase in the number of atoms making up the molecule.

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Homologous

Series

General Formula Example Functional

Group

Straight chain

Alkanes

Alkyl

Alkenes & Cyclic

Alkanes

Alcohols R-OH

Carboxylic acids

Alkyne

Ether CH3CH2O-CH2CH3 R-O-R

Aldehyde

Homologous

Series

General Formula Example Functional

Group

Ketone

Ester

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DISTINGUISH BETWEEN EMPIRICAL,

MOLECULAR & STRUCTURAL FORMULAE

Empirical Formula

This shows the simplest whole-number ratio of the

numbers of each type of atom present in a molecule.

Molecular Formula

This shows the actual numbers of each type of atom

present in a molecule.

Structural Formula

This shows exactly which atoms are bonded together.

It may be shortened or displayed.

DISPLAYED & CONDENSED (SHORTENED)

MOLECULAR FORMULA

Displayed Condensed

CH3CH2OH

In contrast, students, the molecular formula identifies

the number of each type of atom in a molecule, and the

structural formula also shows the structure of the

molecule.

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WRITE STRUCTURAL FORMULAE

1. Give the following for the compounds listed below:

a) C3H8

b) 2-methylpropane

c) C6H14

d) 2,3-dimethylheptane

e) 2,3-dimethylpentane

f) 1-chloro-2,2-dimethylheptane

DETERMINE FORMULAE FROM

EXPERIMENTAL DATA 1. In an unknown molecule, there are 8.276 g carbon and 1.724 g hydrogen.

Determine the empirical formula for the substance.

2. Two isomeric compounds, A and B, containing only carbon, hydrogen and oxygen are subjected to combustion analysis. One gram (1.0 g) of each compound on complete combustion gives 2.3 g of carbon dioxide and 0.93 g water. The relative molecular mass of each compound is 58.

i. Calculate the empirical and molecular formulae of A and B. (6 marks)

ii. Deduce the structural formulae of A and B. (2 marks)

3. A chemist is given 1.08 g of a compound labeled X, and is asked to determine the molecular formula and the empirical formula. Analysis of X by mass spectroscopy gives a relative molecular mass of 1.08. Elemental analysis shows that compound X contains carbon, hydrogen and one other element. Among the products obtained when X is burnt completely in oxygen are 1340 cm3 of CO2 and 448 cm3 of NO2. [Volumes of gases are measured at s.t.p]

4. In an experiment to determine the empirical and molecular formulae of a hydrocarbon, A, the hydrocarbon is completely burnt in excess oxygen and the products collected. A gave 3.52 g of carbon dioxide and 1.62 g of water, on complete combustion. [Mr CO2 = 44; Mr H2O = 18]

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APPLY THE IUPAC RULES TO NAMED

ORGANIC COMPOUNDS Nomenclature of Unbranched Alkyl Groups

If we remove one hydrogen atom from an alkane, we obtain what is called an alkyl group. These alkyl groups have names that end in –yl. When the alkane is unbranched, and the hydrogen atom that is removed is a terminal hydrogen atom, the names are straightforward:

Alkane Alkyl Group

CH3-H becomes CH3-

Methane methyl

CH3CH2-H becomes CH3CH2-

Ethane ethyl

CH3CH2CH2-H becomes CH3CH2CH2-

Propane propyl

CH3CH2CH2CH2-H becomes CH3CH2CH2CH2-

Butane butyl

APPLY THE IUPAC RULES TO NAMED

ORGANIC COMPOUNDS Nomenclature of Branched –Chain Groups

Branched-chain alkanes are named according to the following rules:

1. Locate the longest continuous chain of carbon atoms; this chain determines the parent name for the alkane.

We designate the following compound, for example, as a hexane

because the longest continuous chain contains six carbon atoms:

CH3CH2CH2CH2CH(CH3)CH3

The longest continuous chain may not always be obvious from the way the formula is written. Notice, for example, that the following alkane is designated as a heptane because the longest chain contains seven carbon atoms:

CH3CH2CH2CH2CH(C2H5)CH3

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APPLY THE IUPAC RULES TO NAMED

ORGANIC COMPOUNDS Nomenclature of Branched –Chain Groups

2. Number the longest continuous chain beginning with the end of the chain nearer the substituent.

Applying this rule, we number the two alkanes that we illustrated

previously in the following way:

CH3CH2CH2CH2CH(CH3)CH3 CH3CH2CH2CH2CH(C2H5)CH3

3. Use the numbers obtained by the application of rule 2 to designate the location of the substituent group.

The parent name is placed last, and the substituent group, preceded by

number designating its location on the chain, is placed first.

Numbers are separated from words by a hyphen. Our two examples are 2-methylhexane and 3-methylheptane, respectively:

CH3CH2CH2CH2CH(CH3)CH3 CH3CH2CH2CH2CH(C2H5)CH3

APPLY THE IUPAC RULES TO NAMED

ORGANIC COMPOUNDS Nomenclature of Branched –Chain Groups

4. When two or more substituents are present, give each substituent a number corresponding to its location on the longest chain.

For example, we designate the following compound as 4-ethyl-2-

methylhexane

CH3CH(CH3)CH2CH(C2H5)CH2CH3

The substituent groups should be listed alphabetically (i.e., ethyl before methyl). In deciding on alphabetical order, disregard prefixes such as ‘di’ and ‘tri’.

5. When two substituent groups are present on the same carbon, use that number twice:

CH3CH2C(CH3)(C2H5)CH2CH2CH3

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APPLY THE IUPAC RULES TO NAMED

ORGANIC COMPOUNDS Nomenclature of Branched –Chain Groups

6. When two or more substituents are identical, indicate this by the use of the prefixes di-, tri-, tetra-, and so on.

This make a certain that each and every substituent has a number. Commas are

used to separate numbers from each other.

CH3CH(CH3)CH(CH3)CH3 CH3CH(CH3)CH(CH3)CH(CH3)CH3

CH3C(CH3)2CH2C(CH3)2CH3

Application of these six rules allows us to name most of the alkanes that we shall encounter. Two other rules, however, may be required occasionally:

7. When two chains of equal length compete for selection as the parent chain, choose the chain with the greater number of constituents:

CH3CH2CH(CH3)CH(C3H7)CH(CH3)CH(CH3)2

APPLY THE IUPAC RULES TO NAMED

ORGANIC COMPOUNDS

Nomenclature of Branched –Chain Groups

8. When branching first occurs at an equal distance

from either end of the longest chain, choose the

name that gives the lower number at the first

point of difference:

CH(CH3)2CH2CH(CH3)CH(CH3)2

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APPLY THE IUPAC RULES TO NAMED

ORGANIC COMPOUNDS Nomenclature of Alkenes and Cycloalkenes

The IUPAC rules for naming alkenes are similar in many respects to those for naming alkanes:

1. Determine the parent name by selecting the longest chain that contains the double bond and change the ending of the name of the alkane of identical length from –ane to –ene.

Thus, if the longest chain contains five carbon atoms, the parent name for the alkene is pentene; if it contains six carbon atoms, the parent name is hexene, and so on.

2. Number the chain so as to include both carbon atoms of the double bond, and begin numbering at the end of the chain nearer the double bond. Designate the location of the double bond by using the number of the first atom of the double bond as a prefix. The locant for the alkene suffix may precede the parent name or be placed immediately before the suffix.

CH2=CHCH2CH3 CH3CH=CHCH2CH2CH3

APPLY THE IUPAC RULES TO NAMED

ORGANIC COMPOUNDS 3. Indicate the locations of the substituent groups by the

numbers of the carbon atoms to which they are attached:

CH3C(CH3)=CHCH3 CH3C(CH3)=CHCH2CH(CH3)2

CH2CH=CHCH2C(CH3)3 CH3CH=CHCH2Cl

4. Number substituted cycloalkanes in the way that gives the carbon atoms of the double bond the 1 and 2 positions and that also gives the substituent groups lower numbers at the first point of difference.

With substituted cycloalkanes it is not necessary to specify the position of the

double bond since it will always begin with C1 and C2. The two examples shown below illustrates the application of these rules:

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APPLY THE IUPAC RULES TO NAMED

ORGANIC COMPOUNDS

5. If two identical groups are on the same side of the

double bond, the compound can be designated cis;

if they are on opposite sides it can be designated

trans:

WRITE THE STRUCTURAL (CONDENSED &

DISPLAYED) FORMULA FOR THE FOLLOWING

a) 2,4-dimethyl-2-pentene

b) Trans-1,4-dichloro-2-pentene

c) 4,5-dibromo-1-pentene

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APPLY THE IUPAC RULES TO NAMED

ORGANIC COMPOUNDS Nomenclature of Alcohols

1. Select the longest continuous carbon (carbinol carbon) chain to which the hydroxyl is directly attached. Change the name of the alkane corresponding to this chain by dropping the final e and adding the suffix –ol.

2. Number the longest continuous carbon chain so as to give the carbon atom bearing the hydroxyl group the lower number.

Indicate the position of the hydroxyl group by using this number as a

locant; indicate the positions of other substituents (as prefixes) by using the numbers corresponding to their positions along the carbon chain as locants.

The following examples show how these rules are applied:

CH3CH2CH2OH CH3CH(OH)CH2CH3 CH3CH(CH3)CH2CH2CH2OH

APPLY THE IUPAC RULES TO NAMED

ORGANIC COMPOUNDS Nomenclature of Aldehydes & Ketones

All aldehydes have a carbonyl group , bonded to one side to a hydrogen. In ketones, the carbonyl group is situated between two carbon atoms.

Note that R represents an alkyl group.

In the IUPAC system aliphatic aldehydes are named substitutively replacing the final –e of the named corresponding alkane with –al.

Since the aldehyde group must be at the end of the chain of carbon atoms, there is no need to indicate its position.

When other substituents are present, however, the carbonyl group carbon is assigned position 1.

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APPLY THE IUPAC RULES TO NAMED

ORGANIC COMPOUNDS

NOMENCLATURE OF ALDEHYDES & KETONES

Aldehydes in which the –CHO group is attached to a ring system are named substitutively by adding the suffix carbaldehyde.

Aliphatic ketones are named substitutively by replacing the final –e of the name of the corresponding alkane with –one. The carbon chain is numbered in the way that gives the carbonyl carbon atom the lower possible number, and this number is used to designate its position.

.

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NOMENCLATURE OF ALDEHYDES &

KETONES

Common functional group names for ketones are obtained

simply by separately naming the two groups attached to

the carbonyl group and adding the word ketone as a

separate word.

Some ketones have common names that are retained in

the IUPAC system.