Mechanisms of ionic addition (addition of HCl, Br2, Cl2, only to ethene).

Evidence for this mechanism:

“reaction of ethene with bromine water containing sodium chloride results in the formation of 2-bromoethanol,1-bromo-2-chloroethane and1,2-dibromoethane.”

FACT

(Name the instrument/technique used to determine Mr accurately )When you react ethene with bromine water containing sodium chloride you get the following substances :- 2-bromoethanol,1-bromo-2-chloroethane and1,2-dibromoethane.

We know this from their different Mr.

A hydroxyl (OH) group must have been added (from water) to an intermediate ion to form 2 bromo ethanol (leaving H+ see below)

A Cl- ion must have been added to an intermediate ion to form 1bromo 2 chloro ethane.

Na+ ions are also present. BUT they don’t add on to the intermediate compound.

The addition of the negative ions (Cl- and OH group) but not the positive Na+ show that the intermediate ion has to be positive ( i.e. the carbonium ion). This is because a positive ion will not be attracted to another positive ion as like charges repel. Opposite charges attract so the intermediate must be positive.

NOTE

Due to the large size of a Bromine atom the structure of the intermediate ion during the addition of Br2 (only Br2 not HCl or Cl2) is thought to be the cyclic bromomium ion. There is no cyclic intermediate in the addition of HCl or Cl2 due to the smaller size of the chlorine and hydrogen atoms. The evidence above still applies as the intermediate is still positive.

Cyclic Bromomium ion

Reaction types and Mechanisms Checklist

1. What the syllabus says:-

Students are not, in general, required to know the conditions (temperature, pressure, catalyst, solvent) for these reactions, except where specified elsewhere in this syllabus. They are required to be able to write balanced equations for the reactions, using structural formulas, unless otherwise indicated.

(a) Addition reactions

Alkenes – reactions with hydrogen, chlorine, bromine, water and hydrogen chloride

Industrial sources. Industrial importance of (i) products of the addition reactions of ethene with

chlorine, bromine, water and hydrogen chloride (ii) hydrogenation of vegetable oils.

Mechanisms of ionic addition (addition of HCl, Br2, Cl2, only to ethene).

Evidence for this mechanism: *****PTO *****

reaction of ethene with bromine water containing sodium chloride results in the formation of 2-bromoethanol,1-bromo-2-chloroethane and1,2-dibromoethane.

Polymerisation reaction (of ethene and propene only –reaction mechanism not required).

Unreactivity of benzene with regard to addition reactions, relative to ethene.

(b) Substitution Reactions

Halogenation of alkanes.Mechanism of free radical substitution (monochlorination of methane and ethane only).

Evidence for this mechanism:

(i) use of ultraviolet light even for a very short period causes a chain reaction

(ii) formation of trace quantities of ethane and butane in the monochlorination of methane and ethane,respectively

(iii) these reactions are speeded up by the addition of a known source of free radicals, such as

tetraethyllead.

Alkenes as raw materials in the industrial manufacture of plastics.

An indication of the range and scope of the petrochemical industry (two examples of

synthetic products of this industry; structures not required, unless specified elsewhere in the syllabus).