CHAPTER 12: Unit 5. Chemical Reactions

Learning Objective: Describe how to prepare alcohols from alkenes.Methanol is prepared by combining hydrogen gas and carbon monoxide at high temperatures and pressures in the presence of a catalyst composed of zinc oxide (ZnO) and chromium oxide (Cr2O3) catalyst:Methanol is an important solvent and is used as an automotive fuel, either as the pure liquid—as in some racing cars—or as an additive in gasoline. Preparation of alcohols: A general method for preparing alcohols is the hydration of alkene.Alkenes react with water ( an unsymmetrical addition agent) in the presence of sulfuric acid ( the catalyst) to form an alcohol. Markonikov’s rule is used to determine the most predominant alcohol product.

Example

Write the equation for the reaction of 2-butene with water to form 2-butanol. Indicate that sulfuric acid is used as a catalyst.

Solution

First write the condensed structural formula of 2-butene and indicate that it reacts with water. Then write the condensed structural formula of 2-butanol after the reaction arrow to indicate that it is the product. Finally, write the formula for the catalyst above the arrow.

Another method of synthesizing alcohols involves the addition of H2 to a C-O double bond in carbonyl group.

A carbonyl group behaves like a carbon carbon double bond when it reacts with H2 under proper condition. As a result of H2 addition, the oxygen of the carbonyl group is converted to an -OH group.

Of the many chemical reactions that alcohols undergo, five will be considered in this section:

1. combustion
2. Dehydration
3. Condensation
4. Oxidation
5. Halogenation

Combustion: Hydrocarbons of all types undergo combustion in air to produce carbon dioxide and water. Alcohols are also flammable, as with hydrocarbons and combustion products are CO2 and H2O. Both methanol and theanol are used as alcohol-gasoline mixtures.

Alcohol dehydration: Alcohols can be dehydrated in the presence of sulfuric acid to form alkenes or ethers. At 180⁰C, an alkene is produced; at 140⁰C, the primary alcohols produce an ether. This is an example of condensation reaction.

This reaction is classified as elimination reaction. Elimination is a reaction in which elements of the starting materials are lost and a new multiple bond is formed.

To draw the product of any dehydration, remove the elements of H and OH from two adjacent atoms and draw a carbon-carbon double bond in the product.

Sometimes more than one alkene can be formed as the product of a dehydration reaction, as in the case when 2-butanol [CH3CHOHCH2CH3] is the starting material. If H and OH are removed from C1 and C2, 1-butene is formed. If H and OH are removed from C2 and C3, 2-butene is formed.

Although both products are formed, the major product is the alkene with more alkyl groups bonded to the carbons of the double bond. Therefore 1-butene is the minor product and 2-butene is the major product. This is called Zaitsev’s rule.

Consider the following two different alcohols,

Only product is obtained from the left hand side structure, so that is the correct alcohol. There are two products obtained from the right hand side structures, following the Zaitsev’s rule, most substituted alkene will be major product

IN CLASS PRACTICE PROBLEMS:

Condensation: A condensation reaction is a chemical reaction in which two molecules combine to form a larger one and a molecule of water.A condensation reaction occur in two steps.A water molecule is formed from the renewal of an H atom from one molecule and a hydroxyl group from another molecule, leaving  behind two organic parts.Two organic fragments are then joined together through formation of a covalent bond.There are two important difference between dehydration and condensation reaction.In a condensation reaction, the removed H and OH come from two different molecules whereas in a dehydration reaction the removed H and OH come from the same molecule.In a condensation reaction, the H is removed from O or N atom whereas in a dehydration reaction the H is removed from a C atom. Alcohol condensation produces an ether.Under the proper conditions, it is possible for the dehydration to occur between two alcohol molecules. The entire OH group of one molecule and only the hydrogen atom of the OH group of the second molecule are removed. The two ethyl groups attached to an oxygen atom form an ether molecule. The factor determines whether a dehydration or condensation reaction occur when two alcohols react is temperature.  at lower temperature ether formation is possible while at higher temperature, i.e. 180 0C alkene formation is likely.In practice, condensation reactions produce useful yields of ether only for primary alcohol even at lower temperature. Secondary and tertiary alcohol undergo dehydration instead. https://www.youtube.com/watch?v=pMgVplZe91wAlcohol Oxidation: Oxidation of primary alcohols first produces an aldehyde, which is further oxidized to a carboxylic acid. Secondary alcohols are oxidized to ketones and tertiary alcohols are resistant to oxidation. Oxidation always takes place by removing one Hydrogen from the carbon containing functional group and the hydrogen from the -OH group. Since tertiary alcohol doesn’t have any hydrogen connected to Carbon containing -OH group, therefore they cannot be oxidized. Primary and secondary alcohols, the two types of oxidizable alcohols, yield different products upon oxidation. A 1 degree alcohol produces aldehyde that is often then further oxidized to carboxylic acid and 2 degree alcohol produces a ketone.

There are two ways oxidation reaction occurs.

1. An increase in the number of carbon-oxygen bonds present in the molecule
2. A decrease in the number of carbon-hydrogen bonds present in a molecule

Example:

Write an equation for the oxidation of each alcohol. Use [O] above the arrow to indicate an oxidizing agent. If no reaction occurs, write “no reaction” after the arrow.

1. CH₃CH₂CH₂CH₂CH₂OH
2. 
3. 

Solution

The first step is to recognize the class of each alcohol as primary, secondary, or tertiary.

1. This alcohol has the OH group on a carbon atom that is attached to only one other carbon atom, so it is a primary alcohol. Oxidation forms first an aldehyde and further oxidation forms a carboxylic acid.
2. This alcohol has the OH group on a carbon atom that is attached to three other carbon atoms, so it is a tertiary alcohol. No reaction occurs.
3. This alcohol has the OH group on a carbon atom that is attached to two other carbon atoms, so it is a secondary alcohol; oxidation gives a ketone.

HOMEWORK EXERCISE

Name the three major types of chemical reactions of alcohols.Why do tertiary alcohols not undergo oxidation? Can a tertiary alcohol undergo dehydration?Draw the structure of the product for each reaction.a. b.  Draw the structure of the product for each reaction.a.  b.5. Write an equation for the dehydration of 2-propanol to yield each compound type.an alkenean etherDraw the structure of the alkene formed by the dehydration of cyclohexanol.Answersdehydration, oxidation, and esterification 3.a.   b. 5.a. b.  Ethers: Two specific important reactions of ethers are mentioned below. Unlike alcohols, ethers cannot be oxidized to aldehydes and acids.ethers are flammable. Special care must be taken in laboratories where ethers are used. Diethyl ether, whose boiling point of 350C is only a few degrees above room temp, is a flash-fire hazard.ethers slowly react with oxygen from the air to form unstable hydroperoxides and peroxides. R-O-O-R Thiols:  Thiols are oxidized to disulfides., compounds that contain sulfur-sulfur bond. This is an oxidation reaction where two hydrogen atoms are removed in forming the disulfide. Disulfides are responsible for curly or straight hair. They can be reduced back to original thiols.