279.2-Bromopentane is heated with potassium ethoxide in ethanol. The major product obtained is
1.pent-1-ene
2.cis-pent-2-ene
3.trans-pent-2-ene
4.2-ethoxypentane
Correct ans is 3. Plz exp.
Answers
When alkyl halide reacts with alc. KOH then it favours elimination reaction (Dehydrohalogenation). Since, Trans pentene-2 is more symmetrical than cis isomers. Hence, it is main product.
The product formed is trans-2-pentene.
Alkyl halide on reaction with ethoxide can undergo either substitution or elimination reaction, SN1, SN2, E1 or E2.
The reaction involves a secondary alkyl halide and a strong nucleophile, ethoxide. So, the preferred mechanism is SN2 or E2.
The reaction occurs a protic solvent, ethanol. Ethoxide is an anion and it form hydrogen bonds with the solvent. This makes the nucleophile less reactive as the approach of the electrophile is hindered and this blocks the substitution reaction.
The preferred reaction is beta elimination by E2 mechanism and this occurs via the Zaitsev rule. More substituted alkenes are more stable.
The trans isomer is the preferred one as the bulky alkyl groups are placed far apart.
Hope this information will clear your doubts about beta elimination reaction of alkyl halides