Halogen Derivatives

State the general formula of halogenoalkanes

1. CnH2n+1X, where X is F, Cl, Br or I.

Explain why melting points of halogenoalkanes are higher than alkanes with similar Mr

1. A larger amount of energy is required to overcome stronger pd-pd attractions between polar halogenoalkane molecules than weaker id-id attractions between alkane molecules during melting.

Explain why melting points increases with larger halogen size

1. From CH3Cl to CH3I, size of electron cloud increases, extent of polarisation of electron cloud increases, strength and extensiveness of id-id attractions increases, hence energy required to overcome id-id attractions between molecules increases from CH3Cl to CH3I.

Explain why halogenoalkanes are insoluble in water

1. Formation of id-id attractions between halogenoalkane and water molecules releases insufficient energy to overcome pd-pd attractions between halogenoalkane molecules and hydrogen bonds between water molecules.

Define nucleophiles

1. Nucleophiles are electron pair donors that are usually either negatively charged anions or neutral molecules that possess at least one lone pair of electrons.

State the reactions undergone by halogenoalkanes and their respective reagents and conditions

1. Elimination of HX: NaOH in ethanol, heat/ Ethanolic NaOH, heat/ KOH in ethanol, heat/ Ethanolic KOH, heat.

2. Formation of alcohol: NaOH(aq), heat.

3. Formation of nitrile: KCN in ethanol, heat.

4. Formation of ether: Na in excess alcohol, heat.

5. Formation of ester: Silver(I) salt of carboxylic acid in alcohol, heat.

6. Formation of primary amine: NH3 in ethanol, heat under pressure.

7. Formation of secondary amine: -NH2 in ethanol, heat under pressure.

8. Formation of tertiary amine: -NH in ethanol, heat under pressure.

9. Formation of quaternary amine: -N in ethanol, heat under pressure.

State and explain the trend in reactivity of RX towards hydrolysis

1. Atomic radius of I atom> Br atom> Cl atom> F atom, effectiveness of orbital overlap between C and I< Br< Cl< F, bond strength and bond energy of C-I< C-Br< C-Cl< C-F, hence reactivity of C-I< C-Br< C-Cl< C-F.

Explain why the optical isomerism of the product is reversed for SN2

1. The nucleophile attacks the C atom from the side opposite to the halogen atom.

2. If the C atom is chiral, then the product would have an inversion of configuration compared to the reactant.

Explain why primary RX undergoes SN2 while tertiary RX do not?

1. Primary RX has the least number of bulky R groups around C atom of C-X bond, resulting in the least steric hindrance, hence it is easier for nucleophilic attack on the electron deficient C atom from opposite side of the halogen atom.

Explain why a racemic mixture is obtained when the RX is chiral

1. There is equal probability of the nucleophile attacking the trigonal planar C atom of carbocation from either top or bottom of the plane, forming equal quantities of both enantiomers and hence a racemic mixture is formed.

Explain why tertiary RX undergoes SN1 while primary RX do not

1. Tertiary RX has the greatest number of electron donating alkyl groups, dispersing the positive change on C atom to the greatest extent, hence stabilising the carbocation.

2. In chlorobenzene, SN1 mechanism is favoured even though it is a primary RX due to resonance stabilisation from the delocalisation of pi electrons over the C+ atom.

Describe reactions to distinguish between different halogenoalkanes and halogenoarenes

1. Heat RX with NaOH(aq), add excess dilute HNO3(aq) and add AgNO3(aq).

2. Add ethanolic silver nitrate.

Results:

RCl: white ppt.

RBr: pale cream ppt.

RI: yellow ppt.

RF and halogenoarene: no ppt.

Explain why are fluroalkanes and chlorofluroalkanes are chemically inert

1. Fluoroalkanes and CFCs are chemically inert, stable, non-flammable and low toxicity due to the high bond energy of C-F bond.

Explain the effects of CFCs on the ozone layer

1. CFCs decompose by UV light to produce Cl radicals which can damage the ozone layer.

Describe how the ozone layer is protected and the environmental impacts of using HFC and HCFC

1. HFCs do not produce chlorine radicals, hence it does not contribute to the destruction of the ozone layer. However, some HCFs are powerful greenhouse gases with higher global warming potential.

2. HCFC are more easily degraded than CFCs before they reach the upper atmosphere, hence much of the chlorine in HCFCs does not reach the stratosphere, resulting in less damage to the ozone layer. HCFCs is a less powerful greenhouse gas with lower global warming potential.

Explain the relative ease of hydrolysis of acyl chlorides, alkyl chloride and aryl chlorides

1. Acyl chloride: The carbonyl C atom is attached to 2 electron withdrawing atoms, hence the carbonyl C atom is highly electron-deficient and most susceptible to nucleophilic attack. The groups around the carbonyl C atom are arranged in a trigonal planar arrangement, resulting in least steric hindrance.

2. Alkyl chloride: The alkyl C atom is attached to 1 electron withdrawing atoms, hence the alkyl C atom is less electron-deficient and less susceptible to nucleophilic attack. The groups around the alkyl C atom are arranged in tetrahedral manner, resulting in greater steric hindrance.

3. Aryl chlorides: The lone pair of electron on Cl atom is delocalised into the benzene ring, hence strengthening the C-Cl bond, making it more difficult to break. The bulky benzene ring results in greatest steric hindrance.

4. Therefore ease of hydrolysis of acyl chloride> alkyl chloride> aryl chloride.

Explain why alkenes with halogen do not undergo nucleophilic substitution

1. The lone pair of electrons on Cl is delocalised into the adjacent C=C, hence strengthening the C-Cl bond, making it difficult to break.