Dehydration of Alcohol

The Dehydration of Alcohol:

Dehydration of alcohol is an elimination reaction which is catalysed by acid.

In this reaction, an alcohol is converted into alkene by loosing water in the presence of acid and the application of heat.

The reaction can be carried out in either of two ways.

·        By heating alcohol with sulfuric acid (H₂SO₄) or phosphoric acid (H₃PO₄)

·        By passing alcohol vapour over alumina (Al₂O₃) which acts as an acid) at high temperature

Mechanism:

The reaction takes place in three steps.

1. Reaction between acid and alcohol gives the protonated alcohol and conjugate base of the acid.
2. The protonated alcohol undergoes hydrolysis to form the carbocation and water.
3. The carbocation looses a proton to the base to give alkene.

The rate of dehydration depends upon last two steps; formation of carbocation and loss of proton.

Ease of Dehydration:

The various classes of alcohols differ widely in ease of dehydration. The order of reactivity of alcohols towards dehydration is:

3⁰ > 2⁰ >1⁰

Tertiary alcohols undergo dehydration the most rapily. This is because, they form the most stable carbocations than any other alcohols and once these cations formed they give the most stable alkenes.  

Orientation of the reaction is strongly Saytzeff:

When there is more than one type of β-hydrogens (β1 and β2) in alcohol then there is a possibility of formation of more than one alkene. In such case, preferred alkenes is more stable one, which can be identified by using Saytzeff’s rule. The dehydration of alcohol is strongly oriented to saytzeff rule.

Saytzeff’s rule: According to this rule, the preferred product is that alkene which is formed by removal of the hydrogen from the β-carbon having the fewest hydrogen substitutents.

For example: In dehydration of tert-Pentyl alcohol, two products 2-Methyl-2-butene and 2-Methyl-1-butene are formed.

‘Since there are two types of β carbon (β1 and β2), therefore two alkenes are expected.  

Here β2 is having fewer number of hydrogen than β1, so according to Saytzeff’s rule preferred product is formed by the removal of the hydrogen from β2. Thus 2-Methyl-2-butene is obtained as main product. 

Isoprene and Isoprene Rule

Isoprene: 

• Isoprene (2-methyl-1,3-butadiene)is a colourless volatile liquid. It is an organic compound which formula is CH₂=C(CH₃)CH=CH₂

•   Isoprene is one of the nature’s favourite building blocks. It either alone or in the combination with other unsaturated compound is used to make polymeric compound.

(A polymeric compound is giant molecule which is formed by the combination of many small or similar compound bonded together)

Butyl rubber is one of such example which is formed by isobutene and a small amount of isoprene.

•  It occurs not only in rubber, but in wide variety of compounds isolated from plants and animal source.  

Isoprene Rule:

Terpenes, which are found in the essential oils of many plants, have carbon skeletons made up of isoprene units. In this compound, the isoprene units are joined in the multiples way. This idea leads to the basis of the “isoprene rule".

Linking between two isoprene molecules could occur in three ways:

1         Head to head or 1-1 link

2         Head to tail or 1-4 link

3         Tail to tail or 4-4 link

1)   1-1 or head to head link: In this linking the head of one isoprene could link with the head of another isoprene molecule.  

2)   1-4 or head to tail link: The head of one isoprene molecule could link with the tail of another isoprene molecule.

3)   4-4 or tail to tail link: The tail of one isoprene molecule could link with the tail of another isoprene molecule.

Isoprene rule states that, in most naturally occurring terpenes like Myrcene, Limonene, Retinol etc there are 1-4 link.

Myrcene

Limonene

Retinol or Vitamin A

However, there are some other terpene which show 4-4 link also. They are called irregular terpene. Eg:

β-Carotene

Wolff Kishner Reduction

Wolff-Kishner Reduction:

This reaction is discovered by Ludwig Wolff and Nikolai Kischner in 1912.

It is an organic reaction in which an aldehyde or ketone reduces to an alkane in the presence of hydrazine, base, and thermal condition.

Example:

Mechanism of the reaction:

In general, the mechanism of the reaction involves following steps.

• In the starting, a hydrazone is formed by the condensation of hydrazine with the ketone or aldehyde substrate.

•  The obtained hydazone is deprotonated by alkoxide base.

• Proton transfer steps then result in the formation of a N=N bond.

• Deprotonation of   nitrogen and a rearrangement reaction result in the formation of carbanion and the release of nitrogen gas.

•  The carbanion then picks up a proton from water to regenerate the base catalyst and provides the final alkane product. 

cannizzaro Reaction

What is Cannizzaro Reaction?

The Cannizzaro reaction, named after its discoverer Stanislao Cannizzaro.

In this reaction, two molecules of an aldehyde are reacted with strong base to produce a primary alcohol and a carboxylic acid using a hydroxide base.

•  It is a redox reaction means in this reaction; the aldehyde undergoes both the oxidation and reduction. The oxidation product is a salt of a carboxylic acid and the reduction product is an alcohol.

• Only aldehydes that do not have alpha hydrogen show Cannizzaro reaction.(Alpha hydrogen is the hydrogen atom attached to C next to aldehyde functional group i.e. with  (C* - CHO) like formaldehyde HCHO, acetaldehyde CH₃CHO etc).

Mechanism:

 The reaction begins with hydroxide attack on the carbonyl carbon followed by deprotonation to give a dianion.

This unstable intermediate releases a hydride anion which attacks another molecule of aldehyde. In this process the dianion converts to a carboxylate anion and the aldehyde to an alkoxide.

The alkoxide is more basic than water so it picks up a proton from water to gives alcohol as final product.

On the other hand, the carboxylate which is less basic than water, cannot pick a proton from water. Thus, after acid work up it converts into carboxylic acid. 

Limiting reactant

What is a Limiting Reactant?

Basically A reaction is said to be complete when its reactants are consumed. Limiting reagent is all about this.

In a chemical reaction, the limiting reagent is the first to be completely used up and prevents any further reaction from occurring. This is because the reaction cannot proceed further without it; on the other hand other reactants are present in excess of the quantities required to react with it.

We can understand it like this:

A + B = C

In this reaction if A is in excess quantity and B is in limited one than B is known as limiting reactant. Because B will be first to consumed and in the absence of B, A cannot make C so reaction will stop.

Since this reactant limits the amount of product that can be formed so it is known as limiting reactant. The reaction will stop when all of the limiting reactant is consumed.

Determine the limiting reactant:

In a reaction where two reactants are present, we can identify the limiting reactant by going through these steps.

1)   Balance the chemical equation that describes the reaction

2)   Compare the mole ratios of the amounts of reactants used

3)   Comparing the amount of products that can be formed from each reactant

Balanced the chemical equation that describes the reaction:

A balanced equation helps us to know the proportion of each reactant. It reflects the law of conservation of mass, which states that mass of substances produced (products) by a chemical reaction, is always equal to the mass of the reacting substances (reactants).

For example:

2H₂ + O₂ --> 2H₂O

Now the above chemical reaction states that to make one water atom we need two hydrogen atoms and one oxygen atom.  

Compare the mole ratios of the amounts of reactants used:

By comparing the mole ratio of the reactants, we can find out the exact amount of the reactants used in the reaction. . A mole is equal to 6.023 x 10²³ units of the substance and weighs the same as the molecular weight of that substance. Means

1 mole = molecular weight of substance

 Number of molecule present in 1 mole = 6.023 X 10²³ units

Now Let us observe the following reaction:

2 H₂ + O₂ → 2 H₂O

 This chemical equation implies that 2 mol of dihydrogen (H₂) and 1 mol of dioxygen (O₂) react to form 2 mol of water (H₂O). 

Since the molecular weight of hydrogen is approximately equal to 2 grams, So a mole of hydrogen molecules would also weigh approximately 2 grams and be roughly equal to 6.023 x 10²³ molecules of hydrogen

In the same way, the molecular weight of oxygen - approximately 32 grams –is roughly equal to one mole of oxygen molecules

Comparing the amount of products that can be formed from each reactant:

Once the equation is balanced properly and a clear knowledge of the proportion of each reactant is known is obtained, then it is simple to determine which reactant is the limiting reactant.

 For example: In the balanced equation for making water, we can see that it takes twice as many moles of hydrogen atoms than oxygen atoms to make water. In other words, each oxygen atom requires two hydrogen atoms in order to make water. The hydrogen would run out before the oxygen does, and once that happens, the reaction would come to an end.

 Thus it is clear that hydrogen is limiting reactant of the following reaction.

Let us take some more examples to understand it better:

1)    NH₃ (g) + 5O₂ (g) --> 4NO (g) +  H₂O (g) 

In the following reaction, ammonia reacts with oxygen to give nitrous oxide and water.

To find out limiting reactant in the following reaction, first balance the equation. So it will be:

4NH₃ (g) + 5O₂ (g) --> 4NO (g) + 6 H₂O (g)

To complete the reaction, we need 4 mole of ammonia and 5 mole of oxygen. It is clear that ammonia will be the first reactant to consume completely. Thus ammonia is limiting reactant of the above reaction.

2)    2C₆H₆ + 15O₂ --> 12CO₂ + 6H₂O

In the following reaction, benzene reacts with oxygen to give carbon dioxide and water.

The above reaction is balanced. In this reaction 2 mole of benzene is used to complete the reaction while oxygen is present in excess. Thus benzene will be will be the first reactant to consume completely. Thus benzene is limiting reactant of the above reaction.