2.6- Halogenoalkanes Flashcards
(a)
Elimination reaction
An elimination reaction is one that involves the loss of a small molecule to produce a double bond.
(a)
HBr
eliminated from 1-bromopropane to form propene
1-bromopropane + NaOH → Propene + NaBr + H2O
(b)
Nucleophile
A species with a lone pair of electrons that can be donated to an electron-deficient species.
(b)
Nucleophilic substitution
Nucleophilic substitution is a type of reaction where a nucleophile replaces a halogen attached to a carbon atom. In this case, OH⁻ replaces the halogen on the primary halogenoalkane.
(b)
OH⁻ reaction with a primary halogenoalkane
OH⁻ attacks the carbon bonded to the halogen in the halogenoalkane. It donates a pair of electrons to this carbon, forming a bond. This causes the halogen to leave, taking the electrons from the C–X bond with it. The result is an alcohol and a halide ion.
(c)
Effect of bond polarity on the ease of substitution of
halogenoalkanes
Bond polarity is when the carbon and halogen in the C–X bond share electrons unevenly. The carbon is slightly positive, and the halogen is slightly negative.
(c)
Effect on bond enthalpy on the ease of substitution of halogenoalkanes
Bond enthalpy is the strength of the bond between carbon and the halogen. Strong bonds, like C–F, are harder to break.
(d)
Hydrolysis of halogenoalkanes
The relative ease of hydrolysis of the halogenoalkanes can be explained by the bond strength of the carbon–halogen bond. The rate of hydrolysis increases as follows: iodoalkanes > bromoalkanes > chloroalkanes
The C—I bond is the least polar and the weakest bond, so it is most easily broken and iodoalkanes are the most reactive.
(d)
Hydrolysis/Ag+ (aq) test for halogenoalkanes
AgCl =white
AgBr = cream
AgI = pale yellow
(e)
Uses of halogenoalkanes
Solvents, anaesthetics and refrigerants
(e)
Tight regulation of halogenoalkanes
There is a tight regualtion of the use of halogenoalkanes due to toxicity or adverse environmental effects.
(f)
Adverse environmental effects of CFCs
Chlorofluorocarbons (CFCs) have been used as refrigerants
and in aerosols, but their use has been banned because of
their effect upon the ozone layer.
(f)
The relevance of the relative bond
strengths of C‒H, C‒F and C‒Cl in determining their impact in the upper atmosphere
CFCs are inert due to the high strength of the C—F and C—Cl bonds. In the upper atmosphere, above the ozone (O3) layer, the CFCs are
affected by UV radiation. UV has enough energy to break C—Cl bonds producing highly reactive chlorine radicals. The chlorine radicals react with and break down ozone molecules. The reaction can occur by a number of different mechanisms.
Mechanism
Halogenoalkanes mechanisms
Substitution:
* OH- acts as a nucleophile
* Forms an alcohol
* KOH in water (aq)
Elimination:
* OH- acts a base
* Forms an alkene
* KOH in ethanol (anhydrous)
