3.3.1 Introduction to organic chemistry

Question 1

What is a functional group?

Answer 1

An atom or group of atoms which when present in different molecules causes them to have similar chemical properties.

Question 2

What are homologous series?

Answer 2

e.g. Alkanes, Alcohols
Families of organic compounds with the same functional group and same general formula.
• They show a gradual change in physical properties (e.g. boiling point).
• Each member differs by CH2 from the last.
• Same chemical properties.

Question 3

What are the prefixes and suffixes used when naming compounds containing each functional group?

Answer 3

Alkanes - suffix: -ane
Alkenes - suffix: -ene
Alcohols (OH) - suffix: -ol prefix: hydroxy-
Halogenoalkanes - prefix: -fluoro chloro- bromo- iodo-
Aldehydes (COH) - suffix: -al prefix: formyl-
Ketones (CO) - suffix: -one prefix: oxo-
Carboxylic acids (COOH) - suffix: -oic acid
Esters -yl -oate

Question 4

When compounds contain more than one functional group, how do you decided which is named with a suffix and which with a prefix?

Answer 4

The highest precedence group takes the suffix (and the lowest number on the carbon chain), with all others taking prefix form.

Order of priority highest first:
carboxylic acids > aldehydes > ketones > alcohols > alkenes > halogenoalkanes

Question 5

What are some general rules for naming carbon chains?

Answer 5

• Count the longest carbon chain and name appropriately
• Find any branched chains and count how many carbons they contain
• Add the appropriate prefix for each branch chain
E.g. -CH2 methyl, -C2H5 ethyl, -C3H7 propyl
• The position of the functional group on the carbon chain is given by a number - counting from the end of the molecule that gives the functional group the lowest number. For aldehydes, carboxylic acids & nitriles, the functional group is always on carbon 1.
• When there are 2 or more of the same groups, di-, tri-, tetra-, penta-, hexa- are used.
• Words are separated by numbers with dashes
• Numbers are separated by commas
• If there is more than one functional group or side chain, the groups are listed in alphabetical order (ignoring any di, tri).
• If suffix starts with a vowel, remove -e from stem name, if suffix starts with consonant, leave -e
E.g. propan-1-ol (-e from propane removed), propane-1,2-diol (-e not removed)

Question 6

What are the two ways to break a covalent bond?

Answer 6

1. Homolytic fission:
   Each atom gets one electron from the covalent bond - one headed arrow shows movement of one electron.
   When a bond breaks by homolytic fission it forms two free radicals. Free radicals do not have a charge and are represented by a •
2. Heterolytic fission:
   One atom gets both electrons - two headed arrow shows movement of pair of electrons.
   Heterolytic fission produces ions.

Question 7

What is a free radical?

Answer 7

Represented by a •
A free radical is a reactive species which possesses an unpaired electron.

Question 8

What do curly arrows show in a mechanism?

Answer 8

We use curly arrows in mechanisms to show the movement of an electron pair showing either breaking or formation of a covalent bond.

A curly arrow will always start from a lone pair of electrons or the centre of a bond.

The formation of a covalent bond is shown by a curly arrow that starts from a lone electron pair or from another covalent bond.
The breaking of a covalent bond is shown by a curly arrow starting from the bond.

Question 9

What are structural isomers?

Answer 9

Same molecular formula different structures (or structural formulae).

Structural isomerism can arise from
• Chain isomerism
• Position isomerism
• Functional group isomerism

Question 10

What are chain isomers?

Answer 10

Compounds with the same molecular formula but different structures of the carbon skeleton.

E.g. pentane, 2-methylbutane, 2,2-dimethylpropane

Question 11

What are position isomers?

Answer 11

Compounds with the same molecular formula but different structures due different positions of the same functional group on the same carbon skeleton.

E.g. 1-bromopropane, 2-bromopropane

Question 12

What are functional group isomers?

Answer 12

Compounds with the same molecular formula but with atoms arranged to give different functional groups.

E.g. carboxylic acids and esters, alkenes and cycloalkanes

Question 13

What is stereoisomerism?

Answer 13

Stereoisomers have the same structural formulae but have a different spatial arrangement of atoms.

Alkenes can exhibit a type of isomerism called E-Z stereoisomerism.
E-Z isomers exist in alkenes due to restricted rotation about the C=C bond

E-Z stereoisomers arise when:
(a) There is restricted rotation around the C=C double bond.
(b) There are two different groups/atoms attached both ends of the double bond.

Question 14

How do you name E-Z stereoisomers?

Answer 14

First determine the priority groups on both sides of the double bond.
Priority Group: The atom with the bigger atomic number is classed as the priority atom.

If the priority atom is on the same side of the double bond it is labelled Z.

If the priority atom is on the opposite side of the double bond it is labelled E.

Question 15

What is the effect of E-Z stereoisomerism on physical properties?

Answer 15

E-Z stereoisomers can have differing melting and boiling points.

Z-1,2-dichloroethene
Boiling point = higher
This molecule is polar. The polar C-Cl bonds are on the same side of the molecule. One side of the molecule is slightly negative.
The intermolecular forces are both van der waals and permanent dipole-dipole attractions.

E-1,2-dichloroethene
Boiling point = lower
This molecule is non-polar. The polar C-Cl bonds are on opposite sides of the molecule. The dipoles cancel out.
The intermolecular forces are only van der waals so lower boiling point.