Chemistry Concept: Most Important Questions of Organic Chemistry for Board Exam

Here are few most important and scoring questions for board exams, There are very simple questions which do not require must understanding of the concepts and even if you are struggling with chemistry, can learn them easily.

Although chlorine is an electron withdrawing group, yet it is ortho-, para- directing in electrophilic aromatic substitution reactions. Why?
Ans: Cl is ortho- and para directing because it shows +R effect. Cl is ring deactivating because it shows -I effect.Reactivity is thus controlled by the stronger inductive effect and orientation is controlled by resonance effect.
(Note : Draw the resonance structures also if asked for 3 marks)
What are ambident nucleophiles? Explain with an example.
Ans:Ambident nucleophiles are nucleophiles having two nucleophilic sites. Thus, ambident nucleophiles have two sites through which they can attack.
Eg:
-CN and -NC
NO₂ and -ONO
The treatment of alkyl chlorides with aqueous KOH leads to the formation of alcohols but in the presence of alcoholic KOH, alkenes are major products. Explain
Ans:In aqueous solution, KOH produces OH- ions which act as nucleophiles, and nucleophilic substitution takes place.
In alcoholic solutions, KOH produces alkoxide ions (OR-) which act like a strong base, and elimination takes place.
Haloalkanes react with KCN to form alkyl cyanides as main product while AgCN forms isocyanides as the chief product. Explain
Ans: KCN is ionic, CN^- ion is free to form bonds using C as well as N atom but C-C bond is stronger than C-N bond hence bonding takes place from C side and alkyl cyanides (R-CN) are formed as main product.
AgCN is covalent, , CN^- ion is NOT free. Only N atom is available for bonding and hence isocyanides (R-NC) are formed.
Why is sulphuric acid not used during the reaction of alcohols with KI?
Ans: Since sulphuric acid is an oxidizing agent, it oxidizes HI to produce I₂.
Alkyl halides, though polar, are immiscible with water. Why?
Ans: Because they do not form Hydrogen bonds with water.
Grignard’s reagents should be prepared and stored under anhydrous conditions. Why?
Ans: Because they react with water to form hydrocarbons.
What is racemic mixture?
Ans: Equimolar mixture of two enantiomers (d and l forms of same compound) is called racemic mixture. It is optically inactive.
The dipole moment of chlorobenzene is lower than that of cyclohexyl chloride. Explain.
Ans: Chlorobenzene has resonance due to which its C-Cl bond is shorter. Dipole moment depends on bond length. There is no resonance in case of cyclohexyl chloride.
Chloroform is stored in closed dark brown bottles. Explain.
Ans: Chloroform is stored in dark bottles because it is slowly oxidised by air in the presence of light to an extremely poisonous gas phosgene.
Why are haloarenes are less reactive towards nucleophilic substitution reactions than haloalkanes?
Ans: Due to resonance, there is partial double bond character in C-Cl in case of haloarenes due to which it is difficult to replace the Cl. There is not resonance in case of haloalaknes.(Note : Draw the resonance structures also if asked for 3 marks)
Iodoform has an antiseptic property. Why?
Ans: As Iodoform comes in contact with organic matter of skin it decomposes to give free iodine which acts as an antiseptic.

Why are alcohols soluble in water?
Ans: Because they from hydrogen bonds with water.
Explain why propanol has higher boiling point than that of the hydrocarbon, butane?
Ans: Because propanol has hydrogen bonding.
Alcohols are comparatively more soluble in water than hydrocarbons of comparable molecular masses. Explain this fact.
Ans: Due to hydrogen bonding in alcohols.
Give reason for the higher boiling point of ethanol in comparison to methoxymethane.
Ans: Due to hydrogen bonding in ethanol.
What is common name of phenol?
Ans: Carbolic acid.
Draw the structure and give IUPAC name of picric acid.
Ans: 2,4,4-Trinitrophenol
Explain why is ortho nitrophenol more acidic than ortho methoxyphenol?
Ans: Nitro is electron withdrawing group which increases the acidic nature.
Methoxyis electron withdrawing group which decreases the acidic nature.
Explain how does the –OH group attached to a carbon of benzene ring activate it towards electrophilic substitution?
Ans: Due to resonance.
Give the equations of reactions for the preparation of phenol from cumene.
Ans:
. Write chemical reaction for the preparation of phenol from chlorobenzene
Ans:
Give two reactions that show the acidic nature of phenol.
Ans:
Why is phenol stronger acid than alcohol?
Ans: This is because phenoxide ion is more stable than alkoxide ion. The phenoxide ion is stabilized by resonance. There is no resonance in alkoxide ion.
The alkoxy group activates the benzene ring towards electrophilic substitution. It directs the incoming substituents to ortho and para positions in benzene ring.
Ans:This is due to resonance.
Why phenol is weaker acid than carboxylic acid?
OR
Although phenoxide ion has more number of resonating structures than carboxylate ion, carboxylic acid is a stronger acid than phenol. Why?Ans: The conjugate base of carboxylic acid is carboxylate ion. The conjugate base of phenol is phenoxide ion. Resonance stabilisation is more in carboxylate ion than phenoxide ion. This is because the five resonating structures of phenoxide ions are not equivalent. Hence carboxylic acids are more acidic than phenols
Why is Thionyl chloride preferred in the preparation of alkyl halide using alcohol?
Ans: This is because the by-products SO2 and HCl are escapable gases , and hence alkyl halides are obtained in pure form.
Bromination of phenol takes place even in the absence of Lewis acid (catalyst). Why?
Ans: It is due to the highly activating effect of OH group attached to the benzene ring.
Preparation of ethers by acid dehydration of secondary or tertiary alcohols is not a suitable method. Give reason.
Ans: If secondary or tertiary alcohols are used, due to steric hindrance, alkenes are formed and not ethers

Aldehydes are more reactive than ketones. Why?
Ans: There is more steric hindrance in ketones which make them less reactive.
Would you expect benzaldehyde to be more reactive or less reactive in nucleophilic addition reactions than propanal? Explain your answer.
Ans: Less reactive. Due to resonance.
Cyclohexanone forms cyanohydrin in good yield but 2,2,6-trimethylcyclohexanone does not. Give Reason.
Ans: Due to greater steric hindrance in 2,2,6-trimethylcyclohexanone.
There are two –NH₂ groups in semicarbazide. However, only one is involved in the formation of semicarbazones. Give reason.
Ans: Because one of the NH₂ is involved in resonance with carbonyl group.
Draw structure and IUPAC name of aspirin.

Ans: IUPAC: 2-Acetoxybenzoic acid
During the preparation of esters from a carboxylic acid and an alcohol in the presence of an acid catalyst, the water or the ester should be removed as soon as it is formed.
Ans: To prevent reaction from being reversible
Carboxylic acids have higher boiling liquids than alcohols. Give reasons.
Ans: Carboxylic acids have greater extent of hydrogen boning than alcohol.
Which type of aldehydes undergo aldol condensation?
Ans: Aldehydes having alpha hydrogen..
Why pH of reaction should be carefully controlled while preparing ammonia derivatives of carbonyl compound?
Ans: In strongly acidic medium ammonia derivatives being basic will react with acids and will not react with carbonyl compound. In basic medium OH- will attack carbonyl group. So the pH of a reaction should be carefully controlled.
1. Carboxylic acids do not give reactions of aldehydes and ketones, although contain carbonyl group. Why?
Ans: The carbonyl group in carboxylic acid is not a free group as in aldehydes and ketones. It is involved in resonance. Therefore, carboxylic acids fail to give te characteristic reactions of the carbonyl groups as are given by aldehdes and ketones..
11. Explain why aldehydes get oxidised more readily than ketones.
OR
Why aldehydes are stronger reducing agents than ketones?
OR
Why aldehyde reduces tollens reagent but ketone does not?
Ans: Oxidation of ketones involve breaking of C-C bonds which require greater energy so it is difficult to oxidise them. Hence aldehydes are stronger reducing agent than ketones
12. Name the aldehyde which does not give Fehling’s solution test.
Ans: Benzaldehyde.
13. Why are alpha hydrogen of carbonyl compounds acidic?
Ans: Due to the acidic nature of α-hydrogen of aldehydes and ketones, they undergo a number of reactions. This acidity is because of the strong electron-withdrawing nature of the carbonyl groups and resonance stabilisation of the conjugate base.
Ethylamine is soluble in water whereas aniline is not
Ans: Because ethylamine forms hydrogen bonds with water molecules. In aniline benzene ring creates steric hindrance to hydrogen bonding.
Methylamine in water reacts with ferric chloride to precipitate hydrated ferric oxide.
Ans: Methyl amines is a base so it reacts with water to produce OH^- ions. These OH^- ions convert ferric ion into hydrated ferric oxide.
Although amino group is o– and p– directing in aromatic electrophilic substitution reactions, aniline on nitration gives a substantial amount of m-nitroaniline.
Ans: In acidic medium protonation of aniline takes place to form anilinium ion which is meta directing.
Aniline does not undergo Friedel-Crafts reaction.
Ans: This is due to salt formation of aniline with the lews base used as catalyst. As a result, N of aniline acquires positive charge and hence it acts as a strong deactivating group for electrophilic substitution reactions. Consequently, aniline does not undergo Fridel Craft reaction.
1. Diazonium salts of aromatic amines are more stable than those of aliphatic amines.
Ans: Due to resonance.
19. Gabriel phthalimide synthesis is preferred for synthesising primary amines.
Ans: Gabriel phthalimide reaction gives pure primary amines without any contamination of secondary and tertiary amines. Therefore, it is preferred for synthesising primary amines.
Describe the method for the identification of primary, secondary and tertiary amines
Ans: The three types of amines can be distinguished by Hinsberg test. In this test, the amine is shaken with benzene sulphonyl chloride (C₆H₅SO₂Cl) is the presence of excess of NaOH or KOH.
A primary amine reacts to give a clear solution, which on acidification yields an insoluble compound.
A secondary amine forms an insoluble compound, which remains insoluble even on acidification.
A tertiary amine does not react with the reagent.
Why aromatic primary amines cannot be prepared by Gabriel phthalimide synthesis?
Ans: The success of Gabriel phthalimide reaction depends upon the nucleophilic attack by the phthalimide anion on the organic halogen compound. Since aryl halides do not undergo nucleophilic substitution reactions easily, therefore, arylamines, i.e., aromatic, primary amines cannot be prepared by Gabriel phthalimide reaction.
How do aromatic and aliphatic primary amines react with nitrous acid?
Ans: Both aromatic and aliphatic primary amines react with HNO₂ at 273-278 K to form aromatic and aliphatic diazonium salts respectively. But aliphatic diazonium salts are unstable even at this low temperature and thus decompose readily to form a mixture of compounds.
Why are amines less acidic than alcohols?
Ans: This is because O is more electronegative that N.
Why are primary amines higher boiling point than tertiary amines?
Ans: Due to the presence of two H-atoms on N-atom of primary amines, they undergo extensive intermolecular H-bonding while tertiary amines due to the absence of a H-atom on the N-atom do not undergo H-bonding.
Why are aliphatic amines stronger bases than aromatic amines?
Ans: Due to resonance in aniline, the lone pair of electrons on the nitrogen atom gets delocalized over the benzene ring and thus is less easily available for protonation. Therefore, aromatic amines are weaker bases than ammonia.
Ammonolysis of alkyl halides is not a good method. Explain.
Ans: The process of ammonolysis yields a mixture of primary secondary tertiary amines and quaternary salts. The separation of this amines is a very complicated process and difficult.

Glucose or sucrose are soluble in water but cyclohexane or benzene (simple six membered ring compounds) are insoluble in water. Explain.
Ans: Due to hydrogen bonding.
What are the expected products of hydrolysis of lactose.
Ans: β- D glucose and β- D galactose
How do you explain the absence of aldehyde group in the pentaacetate of D-glucose?
Ans: Pentaacetate of D-glucose in aqueous medium does not form open chain structure and thus when it reacts with NH₂OH, it does not form oxime indicating that there is absence of free aldehyde (−CHO) group.
What are monosaccharides?
Ans: Monosaccharides are carbohydrates which cannot be hydrolysed to smaller carbohydates
What are reducing sugars?
Ans: Carbohydrates which reduce Fehling’s solution or Tollen’s reagent are called reducing sugars.
The two strands in DNA are not identical but are complementary. Explain
Ans: In the helical structure of DNA, the two strands are held together by hydrogen bonds between specific pairs of bases. Cytosine forms hydrogen bond with guanine, while adenine forms hydrogen bond with thymine. As a result, the two strands are complementary to each other.
Starch and cellulose both contain glucose unit as monomer, yet they are structurally different. Explain
Ans: Starch and cellulose both contain glucose unit as monomer, yet they are structurally different because Starch is a polymer of $\alpha$ - glucose while cellulose is a polymer of $\beta$ - glucose.
Glucose doesn’t give 2,4-DNP test. Why?
Ans: because the – CHO group in glucose is involved in hemiacetal formation and thus it is not free.
How can you explain the absence of an aldehyde group in the pentaacetate of D-glucose ?
Ans: Pentaacetate of D-glucose does not react with NH₂OH.
Name the bases present in RNA. Which one of these is not present in DNA ?
Ans: Uracil.
What type of bonding provides stability to a-helix structure of protein ?
Ans: Hydrogen Bonding
Give any one property of glucose that cannot be explained by the open chain structure.
Ans: (i) Glucose does not give Schiff's test and does not produce hydrogensulphite addition product with NaHSO₃, despite having aldehyde group
(ii) The pentaacetate of glucose does not react with hydroxylamine.
What type of linkage is present in Nucleic acids
Ans: Phosphodiester linkage
What do you understand by the glycoside linkage?
Ans: The linkage through which two monosaccharides are joined together to form a molecule of disaccharide is called the glycosidic linkage.
What are essential and non-essential amino acids? Give two examples of each type.
Ans: α-Amino acids which are needed for health and growth of human beings but are not synthesised by the human body are called essential amino acids. For example, valine, leucine, phenylalanine, etc. On the other hand, α-amino acids which are needed for health and growth of human beings and are synthesised by the human body are called non-essentials amino acids. For example, glycine, alanine, aspartic acid, etc
What are peptide bonds?
Ans: Bonds which hold two amino acids together in a protein.
Explain Denaturation of proteins with example.
Ans: When a protein in its native form, is subjected to physical change like change in temperature or chemical change like change in pH, the hydrogen bonds are disturbed. Due to this, globules unfold and helix get uncoiled and protein loses its biological activity. This is called denaturation of protein. Eg. Coagulation of albumin present in the white of an egg.
1. What type of bonding helps in stabilising the α-helix structure of proteins?
Ans: Hydrogen bonding.
How do you explain the amphoteric nature of amino acids.
Ans: Amino acids contain an acidic (carboxyl group) and basic (amino group) group in the same molecule. In neutralise aqueous solution, they each other. The carboxyl group loses a proton while the amino group accepts it. As a result, a dipolar or zwitter ion is formed. In Zwitter ionic form, a-amino acid show amphoteric behaviour as they react with both acids and bases.
How are vitamins classified? Name the vitamin responsible for coagulation of blood.
Ans:
(i) Water soluble vitamins: These include vitamin B-complex (B1 , B2 , B5 , i.e. nicotinic acid), (B1 , B12 , panthothenic acid biotin, i.e. vitamin H and folic acid) and vitamin C.
(ii) Fat soluble vitamins: These include vitamins A, D, E and K. They are stored in liver and adipose (fat storing tissues). Vitamin K is responsible for coagulation of blood
What is the difference between a nucleoside and a nucleotide
Ans: A nucleoside contains a pentose sugar and a nitrogenous base.
A nucleotide contains pentose sugar, a nitrogenous base and a phosphoric acid group also.
Why is sucrose called Invert sugar?
Ans: Sucrose is dextrorotatory. When sucrose undergoes hydrolysis, it breaks down into glucose and fructose Glucose is also dextrorotatory, but fructose is laevorotatory. The laevorotation of fructose is stronger than the dextrorotation of glucose. Therefore, the overall rotation of the mixture changes from dextro (+) to laevo (-) i.e. inversion takes place.
Why vitamin C cannot be stored in our body?
Ans: It is water soluble vitamin. It comes out from our body along with urine.

Chemistry Concept

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Friday, July 19, 2024

Most Important Questions of Organic Chemistry for Board Exam