NEET UG Chemistry Organic Compounds Containing Oxygen MCQs

Important Points

A Organic compounds containing oxygen - I

1. Preparation of alcohols:

→ Monohydric alcohols are prepared by the hydrolysis of alkyl halides with aqueous alkali, hydration of alkenes, hydrolysis of ester, reduction of (alde hydes, ketones, acids and acid derivatives). Grignard reagents is also used to prepare monohydric alcohols.

2. Physical properties of alcohols:

→ The boiling points of alcohols are much higher than comperatively same molecular masses of alkanes, ethers and alkyl halides. This is due to intermo lecular Hbond. For isomeric alcohols, the boiling points are in the order 1⁰>2⁰>3⁰.

→ Due to the formation of H-bond between alcohol and H₂O modecules, alcohol with lower number of carbons are soluble in water.

3. Chemical properties of alcohols:

→ Alcohols exhibit three types of reactions,

(i) Reaction in which O-H bond cleaves

(ii) Reaction in which C-O bond cleaves

(iii) Reaction in which whole molecule of alcohol participate.

→ Victor-Meyer’s test and Lucas reagent are used to distinduish 1°, 2° and 3° alcohols.Oxidation reactions are also used to distiguished between 1°, 2° and 3° alcohols.

4. Preparation of phenol:

→ Phenol is prepare from cumene, diazonium salt, benzene and coal tar.

5. Physical properties of phonol:

→ Phenols have higher boiling point than the corresponding hydrocarbon and aryl halides. This is due to the presence of intermolecular hydrogen bonding.

→ Phenols are more acidic than alcohols because phenoxide ion is stabilised by resonance. The presence of electon withdrawing group like NO₂, increases the acidic strength of phenol and electron donating group like R, decreases the acidic strength of phenol.

6. Chemical properties of phenol:

→ Reaction of phenols are mainly of two types,

(i) Reaction involving OH group

(ii) Reaction involving phenyl group

7. Preparation of ethers:

→ Ethers are mainly prepared by Williamson’s synthesis which involces the heating of alkyl halides with sodium or potassium alkoxides or phenoxides. Dehydration of alcohol at  140⁰c also gives ether.

8. Chemical properties of ethers:

→ (i) Reaction involving cleavage of C-O bond with dilute acid or with HX

(ii) Electrophillic substitution reactions occuring in aromatic ring

B: Organic compounds containing oxygen - II

1. Carbonyl compounds:

group or carboxy group compounds. In aldehydes, the carbonyl group is attached to one hydrogen atom and one alkyl (or aryl or hydrogen atom) group, while in ketones it is attached to one alkyl and one arly group or to two alkyl (or aryl) groups, which may be same or diffrent. If carbonyl group is attached to one hydroxyl group, the compounds are know as carboxylic acids. In carboxylic acid compounds, if the hydrogen of hydroxyl group is substituted by alkyl or aryl group the compounds are known as esters, but if it is substitued by acyl group, the compounds are known as acid anhydrides. If the carbonyl group is attached to chlorine and to amino group the compounds are known as acid chlorides and amides respectively. The general formula of these compounds are expressed as Amide 

2. Structure and nature of carbonyl group:

→  Carbonyl carbon atom is sp² hybridised and form three σ-bonds and one ∏ bond. All the three s-bond lie in same plane having angle 120⁰. The ∏- bond lies both above and below the C-O s bond. Thus the carbonyl carbon, oxygen atom and two atoms which are directly bonded to the carbonyl carbon lie in one plane, and is confirmed by electron diffraction and spectroscopic studies. 

→  Due to higher electronegativity of oxygen atom relative to carbon atom the carbonyl group is polarized and carbonyl carbon becomes electrophile (Lewis acid) and oxygen becomes nucleophile (Lewis base). Carbonyl group is polar in nature and has dipole moments.Aldehydes and ketones have dipole moments 2.3-2.8 D. The resonance structures are as shown below :

3. Physical properties of aldehydes and ketones:

9. Which of the following alcohal on heating with conc H₂SO₄ gives product, which show geometrical isomerism ?

(A) 2,4- dimethyl pentan-3-ol B) 2- methyl butan -2-ol

(C) butan-2-ol (D) all of the above

 

10. Propan -l-ol and propan-2-ol can be distingaished by

(A) oxidation with alkaline KMnO₄ followed by reation with Fehing Solution

(B) oxidation with acidic dichromate followed by reation with Fehling Solution

(C) oxidation by heating with copper followed by reation with Fehling Solution

(D) oxidation with conc H₂SO₄ followed by reaction with Fehling Solution

 

11. Which one of the follwing will most readily be dehydrated in acidic condition ?

 

 

12. The best reagent to convert pent - 3 - en-2-ol into pent-3en-2-one is

(A) acidic permanganate (B) acidic dichromate

(C) chromic anhydride in glacial acetic acid (D) pyridinium chlorochromate

 

13. CH₃ CH₂ OH can not be prepare by which of the following reaction ?

(A) Hydrolysis of ethyl acetate

(B) Hydroboration of ethene followed by oxidation in basic medium

(C) Reaction of ethyl chloride with alcoholic potassium hydroxide

(D) Reaction of ethylacetate

 

14. The most suitable reagent for the conversion of primary alcohol into aldehyde with the same number of carban is

(A) acidified K₂Cr₂O₇ (B) alkaline KMnO₄

(C) acidified KMnO₄ (D) pyridinium chlorochromate