Thursday, January 8, 2009

Alcohols - Conceptual Questions - Answers

1. What are alcohols?
The hydroxy derivatives of aliphatic hydrocarbons are termed alcohols. They contain one or more hydroxyl (OH) groups.

2. What are phenols?
Phenols are organic compounds that have hydroxyl (-OH) group bonded to the aromatic rings.

3. Alcohols are organic compounds obtained by replacing hydrogen atom or atoms from aliphatic hydrocarbons by hydroxyl group or groups.

4. Alcohols contain hydroxyl group.

5. What are mono, di-, and trihydric alcohols?
Monohydric alcohols contain one hydroxyl group.
Dihydric alcohols contain two hydroxyl groups.
Trihydric alcohols contain thee hydroxyl groups.


6. What is the further classification of monohydric alcohols?
Primary, secondary and tertiary alcohols.

7. What is the common system of nomenclature of aliphatic alcohols?
Alcohols are named as alkyl alcohols. The alkyl group attached to the –OH group is named and the word alcohol is added to it.
Ex: Methyl alcohol

8. What is the IUPAC system of nomenclature of aliphatic alcohols?
‘e’ in the parent chain is replaced by ‘ol’
Ex: Methanol
Rules follow for arriving at the names of alcohols

1. The longest continuous chain containing the carbon bonded to the OH group is selected as the parent chain.
2. The carbon atoms in the chain are numbered in such a way that the carbon atom carrying the hydroxyl group gets the lowest number.
3. The position of substituents is indicated by suitable numbers.
Ex: 2-Methyl propan-1-ol

The alcohols with two –OH groups are named as diols and alcohols with three –OH groups are named as triols.

Ex: Ethane – 1,2 – diol, propane -1,2,3-triol

9. What is the common system of nomenclature of aromatic alcohols?
They are called as phenols
10. What is the IUPAC system of nomenclature of aromatic alcohols?

11. Explain chain isomerism in alcohols.
Alcohols containing four or more carbon atoms exhibit chain isomerism in which the isomers differ in the chain arrangement attached to the hydroxyl group – straight or branches.

12. Explain position isomerism in alcohols.
Alcohols containing three or more carbon atoms exhibit position isomerism in which the isomers differ in the position of hydroxyl group.

13. Explain functional isomerism in alcohols.
Monohydric alcohols containing two or more carbon atoms have functional isomers, which differ in the functional groups of the molecule.

14. Discuss the methods of preparation of alcohols.

General Methods

i. preparation from haloalkanes
Haloalkanes when boiled with acqueous KOH or moist silver oxide give alcohols.

ii. By reduction of aldehydes, ketones and esters
The reduction is carried out by reducing agents such as (i) hydrogen in the presence of catalyst such as Ni, Pt or Pd (ii) sodium in the presence of ethyl alcohol (iii) lithium aluminium hydride (LiAlH4) or sodium borohydride (NaBH4).

iii. From Grignard reagents (RMgX)
Grignard reagents react with aldehydes and ketones to form addition products which decompose with dil HCl or dil H2SO4 to give alcohols.

iv. By hydrolysis of esters
Hydrolysis of esters is done with aqueous alkalies.

v. From alkenes
----a). Hydration of alkenes
In indirect process of hydration, alkenes are passed through concentrated H2SO4 to form alkyl hydrogen sulphates, which upon hydrolysis with boiling water give alcohols.

In direct process of hydration, alkenes directly add a molecule of water in the presence of mineral acids as catalysts to form alcohol.

----b). Hydroboration oxidation reduction
Alkenes react with diborane (B2H6) to form trialkylboranes which upon subsequent treatment with alkaline H2O2 to give alcohols.

----c). Oxymercuration – reduction
Alkenes react with mercuric acetate (CH3COO)2Hg [also represented as Hg(OAc)2] to form adducts which upon reduction with NaBH4 in alkaline medium give alcohols.

vi. From aliphatic primary amines
Primary amines react with nitrous acid (NaNO2 + HCl) to form alcohols. Nitrogen gas is evolved in the reaction.

Industrial Methods

i. Hydration of alkenes
Both direct and indirect processes are used (methods given above)

ii. Oxo Process
Alkenes react with carbon monoxide and hydrogen in the presence f cobalt carbonyl [CO(CO)4]2 as catalyst at high temperature and pressure to give aldehydes. The catalytic hydrogenation of aldehydes gives primary alcohol.

iii. Fermentation of carbohydrates
Starch or sugar is broken down into simpler molecules like alcohol by the action of biological catalysts.

iv. Manufacture of methanol
Methanol is prepared from water gas (a mixture of carbon monoxide and hydrogen). Water gas is passed over heated Cr2O3-ZnO catalyst at 673 K under high pressure.

Methanol is also prepared as a by-product during the destructive distillation of wood.

15. What are physical properties of alcohols?
a. Physical state and smell
b. Boiling points
c. Solubility in water

A. Physical state: the lower members are colourless liquids and have a characteristic smell and burning taste.

The higher members(with more than 12 carbons) are colourless wax like solids.

B. Solubility: The lower members are highly soluble in water.

Amongst isomeric alcohols, the solubility increases with branching.

C. Alcohols exists associated molecules due to intermolecular hydrogen bonds.

D. Boiling points: The lowers members have low boiling points.
With the increase in molecular weight, the boiling points keep on increasing gradually.

e. Density: Generally alcohols are lighter than water.

Density of alcohols increases with molecular mass.

F. Alcohols have intoxicating effects.

Methanol is poisonous.
Ethanol is used for drinking purposes.

16. Describe the chemical properties and reactions of alcohols

A. Reactions involving cleavage of oxygen-hydrogen bond.
B. Reactions involving cleavage of carbon - oxygen bond
C. Reactions involving cleavage of both the alkyl and hydroxyl groups

A. Reactions involving cleavage of oxygen-hydrogen bond.

1. Reaction with active metals - acidic character
Alcohols are weakly acidic in nature.

They react with active metals such as sodium, potassium, magnesium, aluminium, etc. to liberate hydrogen gas and form metal alkoxide.

2. Reaction with metal hydrides
Alcohols react with metal hydrides to form alkoxides with the evolution of hydrogen gas.

3. Reaction with carboxylic acids (esterification)
Esterification

Alcohols react with monocarboxylic acids, in the presence of concentrated sulphuric acid or dry HCL gas as catalyst, to from esters. This reaction is known as esterification.


4. Reaction with Grignard reagents.
Alcohols react with Grignard reagents to form hydrocarbons.

5. Reaction with acyl chloride or acid anhydride
When alcohols are treated with acyl chloride or acid anhydride in the presence of bases like pyridine or dimethyl aniline (as catalyst), the hydrogen atom of –OH group is replaced by acyl (RCO-) group forming esters.

B. Reactions involving cleavage of carbon - oxygen bond

1. Reaction with hydrogen halides
Alcohols react with hydrogen halides to form haloalkanes.

2. Reaction with phosphorus halides
Phosphorus halides such as PCl5, Pcl3, PBr3 and PI3 react with alcohols to form corresponding haloalkanes.

3. Reaction with thionyl chloride
In reaction with thionyl chloride in the presence of pyridine, alcohols forms chloroalkances.

Reactions involving cleavage of both the alkyl and hydroxyl groups

1. Acidic dehydration
When alcohol vapour is passed over heated alumina catalyst at 513-523 K, ethers are obtained.

When alcohol vapour is passed over heat alumina catalyst at 633 K, alkenes are obtained.

When alcohols are heated with conc. H3PO4, at 443 K, they get dehydrated to form alkenes.

The ease of dehydration of alcohol follows the order 3>2>1 which is also the order of stability of carbocation.

2. Oxidation
The oxidation of alcohols can be carried out by a number of reagents such as acqueous, alkaline or acidified KMnO4, acidified Na2Cr2O7, nitric acid, chromic acid, etc.

Oxidation of primary alcohol gives aldehydes.
Oxidation of secondary alcohols gives ketones.
It is difficult to oxidize tertiary alcohols.

3. Dehydrogenation
Removal of hydrogen from alcohols is achieved with reduced copper at 573 K.
Primary and secondary alcohols form aldehydes and ketones.
Tertiary alcohols gets dehydrated under the reaction conditions to form alkenes.


17. How do distinguish between Primary, Secondary and Tertiary Alcohols?
i. Lucas test


In this test, an alcohol is treated with an equimolar mixture of concentrated hydrochloric acid and anhydrous ZnCl2 (called Lucas reagent).

ii. Victor Meyer’s test
The given alcohol is treated with red phosphorous and iodine resulting in the formation of corresponding alkyl iodide. The alkyl iodide is treated with silver nitrite to form corresponding nitroalkane. The nitroalkane is reacted with nitrous acid and the resulting solution is made alkaline.

Conclusion: Formation of blood red colour indicates primary alcohol. Blue colour indicates secondary alcohol. A colourless solution indicates tertiary alcohol.

18. How do you convert one alcohol into another alcohol?
i. primary alcohol into secondary alcohol

ii) Secondary alcohol into tertiary alcohol
iii. Primary alcohol into tertiary alcohol
iv) Lower alcohol into higher alcohol (ascent of series)

19. Write a short note on the following alcohols.
a. Methanol
b. Ethanol – Absolute alcohol, Methylated spirit or denatured alcohol, power alcohol, alcoholic beverages

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