Chemistry · Haloalkanes And Haloarenes · NEET
In chlorobenzene the carbon holding chlorine is sp2 and the chlorine lone pair goes into the benzene ring by resonance. This gives the C-Cl bond partial double-bond character, so it is short and strong. The ring is also electron-rich, so a nucleophile (like OH-) is pushed away. Because of this, chlorobenzene stays quiet at room temperature. This is exactly why NEET marks aryl halide hydrolysis as the slowest.
Step 1 (elimination): a strong base removes an H from the carbon next to the C-X, and the halide leaves. This makes a very reactive extra bond between two ring carbons, called benzyne (a triple-bond-like species). Step 2 (addition): the nucleophile adds to either of these two carbons, then picks up an H to give the product. So it is 'eliminate first to make benzyne, then add'.
Benzyne has an extra 'bond' but it is not a normal alkyne triple bond. The ring geometry cannot allow a true straight triple bond, so the extra bond is weak and strained. That strain makes benzyne extremely reactive and short-lived. It forms only for a moment and is immediately attacked by the nucleophile.
Because the strong C-Cl bond and electron-rich ring resist nucleophiles, ordinary conditions do nothing. Only very high temperature and pressure with fused/aqueous NaOH give enough energy to pull off the elimination step and form benzyne. This industrial route is called the Dow process. The harshness itself is the exam clue that haloarenes are unreactive.
Chemists used chlorobenzene with the chlorine carbon labelled with radioactive C-14. After reaction with NaNH2, the amino group (-NH2) appeared equally on the labelled carbon AND the carbon next to it. If it were a simple direct swap, -NH2 would attach only to the labelled carbon. Getting it on both carbons proves a symmetric benzyne intermediate formed in between.
Benzyne (elimination-addition) needs a very strong base like NaNH2 or forcing conditions, and works even without electron-withdrawing groups. If the ring has strong electron-withdrawing groups like -NO2 at ortho/para positions, a different path (addition-elimination) works under milder conditions because the negative charge is stabilised. For NEET, unactivated haloarenes with NaNH2 = benzyne.
The hydrolysis reaction (with aq. NaOH) that takes place at the slowest rate, among the following, is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
SN1 and SN2 happen at an sp3 carbon in alkyl halides by direct substitution. Haloarenes have an sp2 carbon and do not do simple SN1/SN2 easily; instead a strong base makes a benzyne intermediate (elimination), then the nucleophile adds. So it is elimination-addition, not direct substitution.
No. Benzyne is a highly strained, very reactive intermediate that exists only for a moment. It is immediately attacked by any nucleophile present, so it cannot be isolated normally.
A very strong base such as sodium amide (NaNH2) in liquid ammonia, or forcing conditions like fused NaOH at high temperature and pressure, can pull off the elimination step to make benzyne.
Because benzyne forms first. The nucleophile (NH2-) can add to either of the two carbons of the symmetric benzyne bond. This is proven by the C-14 labelling experiment where -NH2 appears equally on the original and neighbouring carbon.
No. Unactivated haloarenes with a strong base use benzyne. But if strong electron-withdrawing groups (like -NO2) sit at ortho/para positions, the ring undergoes an addition-elimination path under milder conditions because the intermediate negative charge is stabilised.
