Hey there! We receieved your request
Stay Tuned as we are going to contact you within 1 Hour
One of our academic counsellors will contact you within 1 working day.
Click to Chat
1800-5470-145
+91 7353221155
Use Coupon: CART20 and get 20% off on all online Study Material
Complete Your Registration (Step 2 of 2 )
Sit and relax as our customer representative will contact you within 1 business day
OTP to be sent to Change
Heating most alcohols is a general method of preoarationof alkenes. Heating alcohols with a strong acid causes them to lose a molecule of water (to dehydrate) and form an alkene.
The reaction is an elimination and is favoured at higher temperatures. The most commonly used acids in the laboratory are Bronsted acids - proton donors such as sulphuric acid and phosphoric acid. Lewis acids such as alumina (Al2O3) are often used in industrial, gas phase dehydrations.
Dehydration reactions of alcohols show several important characteristics which shall be explained.
?The experimental conditions-temperature and acid concentration-that are required to bring about dehydration are closely related to the structure of the individual alcohol. Alcohols in which the hydroxyl group is attached to a primary carbon (primary alcohols) are the most difficult to dehydrate. Dehydration of ethanol, for example, requires concentrated sulphuric acid and a temperature of 180°C.
Secondary alcohols usually dehydrate under milder conditions. Cyclohexanol, for example, dehydrates in 85% phosphoric acid at 165-170°C.
Tertiary alcohols are usually so easily dehydrated that extremely mild conditions can be used, ter-butyl alcohol, for example, dehydrates in 25% H2SO4 at a temperature of 85°C.
Thus, overall, the relative ease with which alcohols undergo dehydration is in the following order
Ease of Dehydration: 3° Alcohol > 2° Alcohol > 1° Alcohol
This behaviour, is related to the stability of the carbocation formed in each reaction.
Some primary and secondary alcohols also undergo rearrangements of their carbon skeleton during dehydration.
Mechanism of Alcohol Dehydration : An E1 Reaction
The mechanism is an E1 reaction in which the substrate is a protonated alcohol (or an alkyloxonium ion). We consider the dehydration of CH3CHOHCH3 that proceeds through a carbonium ion intermediate. A catalytic role is assigned to the acid and O in ROH is a basic site.
Instead of HSO4-, a molecule of alcohol could act as a base in step 3 to give ROH2+.
Because step 2 is then, the rate determining step, it is the step that determines the reactivity of alcohols toward dehydration. The formation of a tertiary carbocation is easiest because the free energy of activation for step 2 of a reaction leading to a tertiary carbocation is lowest. The order of reactivity of the alcohols reflects the order of stability of the incipient carbonium ion (3°>2°>1°).
Get your questions answered by the expert for free
You will get reply from our expert in sometime.
We will notify you when Our expert answers your question. To View your Question
Win Gift vouchers upto Rs 500/-
Register Yourself for a FREE Demo Class by Top IITians & Medical Experts Today !