Halogeno Alkanes

Question 1

what are halogeno alkanes

Answer 1

are saturated organic compounds consisting of carbon, hydrogen and the halogen atoms (F, Cl, I, Br)

Question 2

what are alkyl halides

Answer 2

• A simpler group of halogeno alkanes
In this group there is only 1 halogen atom

Question 3

general formula of alkyl halides

Answer 3

Cn H2n+1 X

Question 4

characteristics of alkyl halides

Answer 4

Most of the alkyl halides are colourless liquids

For a series of corresponding (same number of C atoms) alkyl halides, the boiling point increases from the fluoro compound (Fl) - the iodo compound (I) due to stronger intermolecular forces as the halogen atom becomes bigger

Question 5

3 types of alkyl halides

Answer 5

1. primary
2. secondary
3. tertiary

Question 6

what are primary alkyl halides

Answer 6

• one in which the carbon atom which has the halogen atom is bonded to two hydrogen atoms

R CH2 X

Question 7

what are secondary alkyl halides

Answer 7

the carbon atom which is bonded to the halogen must have:
- only 1 H atom
- 2 alkyl groups (may be identical or different)

Question 8

general formula for secondary alkyl halides

Answer 8

R3 CHX R’

Question 9

what are tertiary alkyl halides

Answer 9

• the carbon atom bonded to the halogen has no hydrogen bonded to it
• this carbon atom is bonded to 3 alkyl groups

Question 10

what is the general formula for tertiary alkyl halides

Answer 10

R3 C X

Question 11

what is nucleophilic substitution

Answer 11

the negatively charged nucleophiles (halogen atom) is attracted to the C atom bonded to the halogen since it has a partial positive charge

the alkyl halides are saturated componds so the nucleophiles will substitute the halogen compound

Question 12

two types of mechanisms of nucleophilic substitution

Answer 12

Sn1 mechanism for tertiary alkyl halides
Sn2 mechanism for primary alkyl halides

Question 13

Sn2 Mechanism

Answer 13

In the slow step, the nucleophile approaches the partially positive C atom, the C-X bond becomes longer and weaker
then an unstable intermediate is formed in which the nucleophile and halogen are partially bonded to the same C atom

in the fast step, the unstable intermediate quickly breaks down into the substituted product
the C-X bond is broken and C-OH bond is completely formed

Question 14

Sn1 Mechanism for Tertiary Alkyl Halides

Answer 14

in the slow step, the C-X bond is broken and a positively charged intermediate (carbocation) is formed

in the fast step, the carbocation quickly bonds to the nucleophile to complete the reaction

Question 15

what happens when nucleophilic substitution occurs

Answer 15

the C-Halogen bond must be broken
since the bond becomes weaker from C-F to C-I, the iodo compounds will be the most reactive since it is easiest to break

Question 16

reaction of NaOH hydrolysis

Answer 16

when an alkyl halide is heated under reflux with NaOH, the OH- substitutes the halogen and an alcohol is formed

Question 17

what method is used when the substance being produced is volatile

Answer 17

the impure mixture is heated in a reaction vessel which is attached to a condenser

Question 18

steps in conversion to a nitrile

Answer 18

when bromoethane is heated under reflux with aqueous KCN or NaCN, the CN nucleophile substitutes the Br atom and a nitrile will be formed

in the reaction, an extra carbon atom is introduced into the organic compound

Question 19

conversation of an alkyl halide to an amine

Answer 19

when iodoethane is reacted with ammonia, the amide nucleophile substitutes the halogen to produce an amine

the reaction is done by mixing the alkyl halide with excess ammonia in a sealed tube and heating to 100 deg C

Question 20

preparation of alkyl halides

Answer 20

The alkyl halides are prepared by passing the halogen hydride through the alcohol, boiling under reflux in the presence of ZnCl catalyst

Question 21

preparation of alkyl chlorides

Answer 21

they can be prepared by heating an alcohol under reflux with PCl5 or SOCl2

the Cl substitutes the OH and HCl fumes are evolved