Pearson BTEC Applied Science (Distinction) Unit 14: Applications of Organic Chemistry Assignment 14A: Understand the structures, reactions, and properties of functional group compounds

H.
BTEC L3 EXTENDED DIPLOMA IN APPLIED SCIENCE
Unit 14: Applications of Organic Chemistry
Assignment 14A: Understand the structures, reactions,
and properties of functional group compounds

Functional group compounds

A functional group refers to an atom/group of atoms within a molecule that
causes chemical properties of the molecule (1). This is relevant to the reactivity
of organic compounds as functional groups are responsible for chemical
properties displayed by organic compounds, such as their reactivity.

Halogenoalkanes

Structure

Halogenoalkanes, also referred to as haloalkanes, are non-carbonyl compounds
(compounds without a carbonyl group, C=O) with an alkyl halide (alkane with a
halogen atom) functional group, e.g., C-Cl, C-Br, etc. Figure 1- 3D model of a
haloalkane (chloromethane) (3)
The structure of halogenoalkanes is very similar to
alkanes except in haloalkanes a hydrogen atom is
replaced by a halogen atom (2). In terms of
nomenclature, the suffix for haloalkanes is -ane
while the prefix can be fluoro/chloro/bromo/iodo
depending on the halogen attached- see Figure 1.

Haloalkanes are alkanes with at least one halogen
atom in place of a hydrogen atom (4)- in Figure 2,
there is a halogen on each molecule shown, making them all haloalkanes.
Figure 2- Displayed formulae of haloalkanes (4)




Reactions

The reactions haloalkanes undergo are (4):
- Nucleophilic substitution, where a nucleophile reacts with a polar molecule
by substituting its functional group
- Elimination, where a small group of atoms breaks away from a molecule

In a nucleophilic substitution reaction, a functional group is substituted for
another using a nucleophile- an electron pair donor consisting of a lone pair of
electrons to form a covalent bond. The most common nucleophiles that react
with haloalkanes are the cyanide ion (CN-), hydroxide ion (OH-) and ammonia
(NH3).
Figure 3- Nucleophilic substitution of cyanide with bromoethane (4)

1

, H.
BTEC L3 EXTENDED DIPLOMA IN APPLIED SCIENCE
Unit 14: Applications of Organic Chemistry
Haloalkanes react with
cyanide to form nitriles
via nucleophilic
substitution and this
reaction occurs when a
haloalkane, e.g.,
bromoethane, is heated
under reflux with ethanolic sodium/potassium cyanide (sodium/potassium
cyanide dissolved in ethanol) (4) (5). Firstly, see Figure 3, the carbon on the
cyanide ion has a lone pair, causing it to be strongly attracted to the + charge
on the carbon so it moves towards it to form a bond (5). The carbon-bromine
bond breaks due to the cyanide ion and the electrons in the carbon-bromine
bond are transferred to the bromine, forming a bromide ion (5). As a result,
bromine leaves and the cyanide ion substitutes it, forming a nitrile, e.g.
ethanenitrile.
Figure 4- Nucleophilic substitution of hydroxide with bromoethane (4)
Haloalkanes also
react with hydroxides
to form alcohols via
nucleophilic
substitution and this
reaction occurs when
a haloalkane, e.g.,
bromoethane, is heated under reflux with aqueous sodium/potassium hydroxide
(5). This reaction starts with the lone pair on the hydroxide ion moving towards
the + carbon as it is strongly attracted to the positive charge, forming a bond-
see Figure 4. The carbon-bromine bond breaks due to the hydroxide ion, causing
bromine to leave, taking both electrons to become a bromide ion (4). A bond is
formed between the carbon and hydroxide ion which results in the formation of
an alcohol, e.g., ethanol.

Figure 5- Nucleophilic substitution of ammonia with bromoethane (4)
Ammonia can react with
haloalkanes to form
amines via nucleophilic
substitution and this
reaction occurs when a
haloalkane, e.g.,
bromoethane, is heated
with excess ethanolic
ammonia (ammonia dissolved in ethanol). As two species are involved, this
reaction is regarded as an SN2 reaction. There are two stages in this nucleophilic
substitution reaction- see Figure 5. In the first stage, the lone pair on the
nitrogen moves towards the + carbon as it is strongly attracted to it, forming a
bond with it which breaks the carbon-bromine bond (5). The bromine leaves and
becomes a bromide ion, taking both electrons. A bond is formed between carbon
and ammonia and then following this the next stage can start. In the second
stage of this reaction, an ammonia molecule reacts with this molecule formed
from the previous stage, e.g., ethylamine, and removes a hydrogen ion from the
nitrogen- this stage of the reaction is reversible (5). This forms an amine and an
ammonium ion (NH4+); the ammonium ion can react with the bromine ion to form
ammonium bromide (CH3CH2Br + 2NH3 -> CH3CH2NH2 + NH4Br) and the amine

2