Functional Group Compounds
Part 1: Reactions of Organic Compounds
Question 1: Halogenoalkanes and their reactions
Halogenoalkanes are a homologous series of molecules which consist of carbon, hydrogen and
halogen atoms. Carbon atoms have a higher electronegativity than halogen atoms, which results in a
polar molecule where there is a slightly positive charge on the carbon atom and a slightly negative
charge on the attached halogen atom. This slight positive charge on the carbon attracts negatively
charged nucleophiles – electron rich molecules which donate a pair of electrons. When the carbon is
fully saturated, meaning it has made all four of its possible bonds, one group will be removed,
known as the leaving group, which results in the nucleophilic substitution reaction.
Halogenoalkanes increase in reactivity down the group, due to the increasing bond length which
allows bonds to be broken more easily; in this case, that would mean that the nucleophile can
substitute the halogen group more easily. Since iodine is lower in the group than chlorine, it forms
longer bonds which are broken more easily, making iodoalkanes more reactive than chloroalkanes
(1).
Figure 1: Nucleophilic Substitution Reaction of 1-Chloropropane and NaOH
Figure 1 above shows the nucleophilic substitution reaction between 1-chloropropane and aqueous
sodium hydroxide. This reaction requires sodium hydroxide to be dissolved in a solvent so that the
hydroxide ion is free to move. It also requires heat to be added, so it is usually done using a reflux
system, to prevent the loss of reagents whilst heating.
Figure 2: Mechanism of Nucleophilic Substitution Reaction of 1-Chloropropane and NaOH
Figure 2 shows the mechanism of the nucleophilic substitution reaction which is shown in figure 1.
This reaction is in two steps; step one involves the polar bond between carbon and the chloride
group breaking, forming a carbocation and negative chloride ion. In step two, the OH nucleophile
from aqueous sodium hydroxide attacks the carbocation, forming propan-1-ol.
, Figure 3: Nucleophilic Substitution Reaction of 2-bromobutane and Ammonia
Figure 3 above shows the nucleophilic substitution reaction between 2-bromobutane and ammonia.
The 2-bromobutane must be heated with a concentrated solution of ammonia in ethanol. This
reaction must take place in a sealed tube to prevent ammonia gas from escaping as it has a boiling
point of -33°C so would remain a gas even in the Leibig condenser (2).
The nucleophilic substitution reaction occurs when the halogen is in the primary or secondary
position. When the halogen is in the secondary or tertiary position, the halogen atom may be expelled
from the molecule, in a reaction known as the nucleophilic elimination reaction (3). When it is in the
secondary position, a higher concentration of ethanol in the solvent, higher temperatures and higher
concentrations of sodium/potassium hydroxide solutions all mean that the reaction is more likely to
be an elimination reaction rather than substitution. These conditions encourage the elimination of the
halogen over the substitution because there is more energy and more concentrated solvent in the
reaction, allowing it to expel the halogen atom without the need to replace it.
Figure 4: Nucleophilic Elimination Reaction of 2-bromopropane and NaOH
Figure 4 shows the nucleophilic elimination reaction between 2-bromopropane and sodium
hydroxide. In the nucleophilic elimination reaction, the halogen atom is expelled and forms an anion
with a lone pair of electrons, leaving the carbon free, unlike the substitution reaction, in which the
nucleophile bonds with the carbon atom in place of the halogen atom. This occurs when the hydroxide
ion attacks a hydrogen ion, which it has a tendency to react with to form water. Because of this, the
electrons must rearrange, expelling the halogen atom and forming a double bond between the carbon
which it was bonded to and the adjacent carbon. The products of this reaction are an alkene, a halogen
anion with a lone pair of electrons, and a water molecule. This mechanism can be seen in figure 5
below.
Figure 5: Mechanism of Nucleophilic Elimination Reaction
