Question

(i) Explain the nature of the C-X bond. Give an example of a compound having sp³ C-X bonding. (ii) Write chemical equations of the following: (a) Carbylamine reaction (b) Hinsberg reaction.

Hint:

The C-X bond is polar covalent (C is delta plus, X delta minus); use an sp³ haloalkane such as CH₃Cl, and recall carbylamine uses CHCl₃ with alcoholic KOH while Hinsberg uses C₆H₅SO₂Cl.

Answer 1

Step 1: Nature of the C-X bond.
In a haloalkane the carbon-halogen (C-X) bond is a polar covalent bond. A halogen atom is more electronegative than carbon, so the shared electron pair is pulled towards the halogen. Carbon becomes slightly positive (delta plus) and the halogen slightly negative (delta minus). This polarity makes the carbon a target for nucleophiles.
Step 2: Trend in the bond.
Electronegativity falls in the order F > Cl > Br > I, so C-F is the most polar. However the size of the halogen grows down the group, so C-X bond length increases and bond strength (bond enthalpy) decreases from C-F to C-I.
Step 3: Example with sp³ C-X bonding.
In chloromethane, CH₃Cl, the carbon forms four sigma bonds and is sp³ hybridised, so the C-Cl bond is an sp³ C-X bond. Chloroethane, CH₃CH₂Cl, is another example.
Step 4: (a) Carbylamine reaction.
A primary amine heated with chloroform and alcoholic KOH gives a foul smelling isocyanide (carbylamine). Only primary amines respond.
CH₃NH₂ + CHCl₃ + 3KOH → CH₃NC + 3KCl + 3H₂O
Step 5: (b) Hinsberg reaction.
An amine reacts with benzenesulphonyl chloride (Hinsberg reagent). With a primary amine such as ethylamine it forms N-ethylbenzenesulphonamide and HCl.
C₆H₅SO₂Cl + H₂NC₂H₅ → C₆H₅SO₂NHC₂H₅ + HCl
The product still has an acidic N-H and dissolves in alkali, which is how a primary amine is recognised.