Halogenoalkanes - Organic Chem I

Question 1

What is the general formula for halogenoalkanes:

Answer 1

CnH2n+2X

Question 2

Compare CH3Cl (chloromethane) with methane:

Answer 2

CH3Cl has a higher boiling point - polar molecule because it’s unsymmetrical, has permanent dipole-dipole forces as well as London dispersion forces. Stronger London forces because each CH3Cl molecule has more electron than each CH4 molecule therefore more energy needed to overcome the stronger forces between the molecules. CH4 only has London dispersion forces

Question 3

Is chloromethane water-soluble?

Answer 3

No, because CH3Cl cannot form hydrogen bonds with H2O molecules

Question 4

Why is ethanol added?

Answer 4

Halogenoalkanes are insoluble in water. Using ethanol ensures that the halogenoalkane dissolves so it can react with the water molecules

Question 5

What is the nucleophilic substitution reaction between halogenoalkanes and potassium cyanide (KCN)?

Answer 5

Heating under reflux
In aqueous ethanol
- Increases the length of the carbon chain
Eg/ CH3CH2Cl + CN- –> CH3CH2CN + Cl-

Question 6

What is the nucleophilic substitution reaction between halogenoalkanes and excess ammonia, NH3(eth)?

Answer 6

• Concentrated solution of ammonia in ethanol
• Sealed tube
• CANNOT be carried out by heating under reflux because the ammonia gas would escape out through the condenser
  Eg/ CH3CH2Cl + 2NH3 –> CH3CH2NH2 + NH4Cl

Question 7

What is the elimination reaction between halogenoalkanes ethanolic alkali?

Answer 7

CH3CH2Cl + OH- (eth) –> C2H4 + H2O + Cl-

Question 8

What do all halogenalkanes have?

Answer 8

A polar carbon-hydrogen bond

Question 9

What is a nucleophile?

Answer 9

A nucleophile is a species capable of donating a lone pair of electrons to form a new dative covalent bond
- Nucleophiles can either be negatively charged

Question 10

What does the Sn1 mechanism (substitution, nucleophilic, unimolecular) look like?

Answer 10

Favoured by 3° halogenoalkanes

Question 11

Why is it described as a unimolecular reaction?

Answer 11

Because the rate determining step (rds) only involves one molecule.

Question 12

Why is this mechanism favoured by 3° halogenoalkanes?

Answer 12

Because the carbocation intermediate is relatively stable.

Question 13

Why is the first step of the mechanism slow?

Answer 13

Because it involves the breaking of strong covalent bonds

Question 14

Why is the second step fast?

Answer 14

Because the two oppositely charged ions are attracted to each other

Question 15

What does an Sn2 reaction (substitution, nucleophilic, bimolecular) look like?

Answer 15

• Favoured by primary halogenoalkanes
  This is a one step mechanism (as no intermediate is involved)

Question 15

What does an Sn2 reaction (substitution, nucleophilic, bimolecular) look like?

Answer 15

• Favoured by primary halogenoalkanes
  This is a one step mechanism (as no intermediate is involved)

Question 16

Why are there two different nucleophilic substitution mechanisms?

Answer 16

SN1: The formation of the carbocation is the greatest barrier in Sn1. So tertiary halogenoalkanes form the most stable carbocations

SN2: Steric hindrance is the barrier in SN2 reactions since the reaction happens when the nucleophile attacks the C at 180 to the C-X bond. Alkyl groups are bulkier than H atoms so when more alkyl groups are attached to the C, it slows down the reaction