Haloalkanes

Question 1

Haloalkanes/ Halogenalkanes

Answer 1

• Organic compounds where at least one of the HYDROGEN atoms has been replaced with a HALOGEN atom.
• General formula ~ CnH2n+1X, where X is a halogen

Question 2

Polarity of Carbon-Halogen bonds

Answer 2

• Halogens are MORE ELCTRONEGATIVE than carbon
• The electron pair in the Carbon-Halogen bond is CLOSER to the halogen atom.
• The Carbon atom is delta + and the halogen is delta -
• All C-X bonds are POLAR
• The C-C and C-H bonds are NON-POLAR as C&H have a very similar electronegativity

Question 3

What would happen if a OH- ion came into contact with a haloalkane?

Answer 3

• The OH- ion would ATTACK the electron deficient CARBON atom.
• The halogen would be replaced with the OH group.(substitution)
• The OH- ion is a NUCLEOPHILE as it DONATES a pair of electrons to the carbon.

Other possible nucleophiles:
- H2O
- NH3
- CN-

Type of reaction :
NUCLEOPHILIC SUBSTITUTION

Question 4

Hydrolysis reaction

Answer 4

• Where WATER , OH-(aq) ions or H+(aq) break a compound into two.

HALOALKANE ——————— ALCOHOL

Example ~ chloroethane(aq) with sodium hydroxide

CH3CH2Cl(aq) + NaOH(aq)—CH3CH2OH(aq) + NaCl(aq)

• NUCLEOPHILIC SUBSTITUTION

REAGENTS & CONDITIONS:
- NaOH/KOH
- Heat under reflux

Question 5

Mechanism for nucleophilic substitution in 3-D:

Hydrolysis of chloroethane with sodium hydroxide

Answer 5

• C-Cl bond is POLAR, carbon is delta+ and chlorine is delta-.
• The OH- ion is ATTRACTED to and ATTACKS C delta+.
• OH- behaves as a NUCLEOPHILE and used a lone pair of electrons to form a COVALENT bond to C delta+
• As the C-O bond FORMS, the C-Cl bond BREAKS
• an electron pair moves on to Cl (HETEROLYTIC FISSION)

PRODUCTS ~ Ethanol + chloride ion

• The groups around the carbon atom attacked by the OH- ion are INVERTED.
• Na + is a SPECTATOR ION so is not included in the mechanism.

Question 6

Method for comparing the rates of hydrolysis of haloalkanes

Answer 6

1. Hydrolyse the haloalkane using ETHANOL so it becomes soluble in water.
2. In a separate test tube, add silver nitrate solution (AgNO3).
3. Add both of these into a water bath of 60 degrees.
4. Add the silver nitrate to the haloalkane mixture and start the stopwatch.
5. Stop the timer when the precipitate has formed.

Question 7

Results of the hydrolysis of haloalkanes practical and what this shows with equations

Answer 7

Iodine:
- 3-5 seconds
- yellow ppt

Chlorine:
- After 20 minutes
- white ppt

Bromine:
- 3-4 minutes
- cream ppt

IODINE is the most reactive as the yellow ppt formed the quickest ~ LOWEST BOND ENTHALPY.

General equation:
CHX + H2O —– CHOH + X- + H+

Precipitation reaction:
X-(aq) + Ag+(aq) —— AgX(s)