Class 12 Chemistry Chapter 9 Amines

Key Concept: Comparative Reactivity

a) Formamide $(HCONH_2)$
[Solution Description] The reduction rate of amides by $LiAlH_4$ generally depends on the steric hindrance and electronic effects present in the molecule. Formamide is the simplest amide with the least steric hindrance, as it contains only one hydrogen directly attached to the carbonyl group. This makes formamide more accessible for nucleophilic attack by the hydride ion from $LiAlH_4.$

Key Concept: Basic Nature

c) It forms an ammonium salt.
[Solution Description] When an amine reacts with an acid, it forms an ammonium salt, showing its basic nature by accepting protons from the acid.

Key Concept: IUPAC Naming

c) N-methylethanamine

[Solution Description] The given name N-methylethanamine refers to a secondary amine. The structure comprises an ethyl group and a methyl group both attached to the nitrogen atom. According to IUPAC nomenclature rules for secondary amines, substituents attached to nitrogen are indicated with ‘N’. Hence, the IUPAC name itself is N-methylethanamine.

Key Concept: Acylation Reaction

b) Methylbenzamide
[Solution Description] Methanamine reacts with benzoyl chloride resulting in the formation of methylbenzamide along with the release of HCl as shown by the equation

$CH_3NH_2 + C_6H_5COCl \rightarrow CH_3NHC(O)C_6H_5 + HCl.$

Key Concept: Physical Properties

a) Primary amine
[Solution Description] Primary amines have the highest boiling points due to their ability to form more extensive hydrogen bonding networks compared to secondary and tertiary amines. Hydrogen bonds significantly increase boiling points, hence primary > secondary > tertiary in terms of boiling point order.

Key Concept: Diazotisation Process

a) Aniline and nitrous acid
[Solution Description] Benzenediazonium chloride is prepared through the reaction known as diazotisation. This involves treating aniline $\text{C}_6\text{H}_5\text{NH}_2$
, a primary aromatic amine, with nitrous acid $(HNO_2)$ in the presence of hydrochloric acid $(HCl).$ The hydrochloric acid provides the $Cl^-$ ion forming the diazonium salt.

Key Concept: Basic Concept

b) Ortho and Para
[Solution Description] The $-NH_2$ group is an ortho and para directing group due to its electron-donating effect which increases electron density at these positions.

Key Concept: Naming Diazonium Salts

d) Benzenediazonium chloride
[Solution Description] The compound $C_6H_5N_2^+Cl^-$ consists of a phenyl group benzene ring, $C_6H_5$ attached to a diazonium ion $(N_2^+Cl^-).$ The correct naming convention for this compound is Benzenediazonium chloride.

Key Concept: Amines Basics

b) Pyramidal structure
[Solution Description]
Amines are derivatives of ammonia and have a pyramidal structure due to the lone pair of electrons present on the nitrogen atom, causing a trigonal pyramidal geometry.

Key Concept: Reaction Conditions

b) Bromine and sodium hydroxide
[Solution Description] The Hoffmann Bromamide Degradation Reaction requires the presence of bromine and a strong base, typically sodium hydroxide, in either an aqueous or ethanolic solution. These reagents facilitate the deprotonation and subsequent reactions leading to amine production.

Key Concept: Concept Integration, Advanced Mechanism

a) Predominantly $C_6H_5NH_2$
[Solution Description] The reaction between aniline $(C_6H_5NH_2)$ and acetic acid $(CH_3COOH)$ leads to the formation of the amine salt $C_6H_5NH_3^+$ and acetate ion $CH_3COO^-.$ However, as both reactants are weak acids and bases, we need to consider their relative strengths. Aniline is a weaker base compared to ammonia due to the electron-withdrawing effect of the phenyl group, making it less effective in accepting protons. Acetic acid is a moderately strong acid among carboxylic acids but not as strong as $HCl.$ At equilibrium, since we have equimolar concentrations, the mixture will contain significant amounts of unreacted $C_6H_5NH_2,$ some $CH_3COOH,$ formed $C_6H_5NH_3^+,$ and $CH_3COO^-$ However, the higher equilibrium concentration will be of the non-reacted species due to low reactivity.

Key Concept: Solubility Comparison

b) Butan-1-ol
[Solution Description] Solubility in water is influenced by hydrogen bonding capability. Amines can form hydrogen bonds with water molecules, which increases their solubility. For smaller amines and alcohols, both are quite soluble due to hydrogen bonding. However, butan-1-ol generally shows slightly higher solubility compared to butan-1-amine because alcohols tend to have stronger hydrogen bonding interactions.

Key Concept: Application of Reaction

b) Hexanenitrile to 1-aminopentane
[Solution Description]
Hexanenitrile to 1-aminopentane involves the reduction of a nitrile to a primary amine by increasing the carbon chain by one atom as depicted in the general reaction:

$R-C \equiv N + 4[H] \rightarrow R-CH_2-NH_2.$

Key Concept: Benzoylation Reaction

c) N-phenylbenzamide
[Solution Description] Benzoylation is a specific form of acylation where benzoyl chloride reacts with an amine like aniline to produce an amide called N-phenylbenzamide (also known as benzanilide).

Key Concept: Stability Explanation

c) Due to resonance stabilization
[Solution Description] Arenediazonium salts exhibit resonance stabilization, which is more effective at low temperatures (273-278 K). The resonance structures allow delocalization of the positive charge over the aryl ring and the diazo group, thereby stabilizing the ion. This resonance effect contributes significantly to the stability of these salts in cold solutions.

Key Concept: Decomposition Analysis

c) Loss of nitrogen gas
[Solution Description] The primary cause for the decomposition of benzenediazonium chloride in the dry state is the loss of nitrogen gas. When these salts are dry, they are more prone to breaking down and releasing nitrogen due to their unstable nature without the stabilizing influence of a solvent like water.

Key Concept: Reaction Mechanism

c) Ammonium ion
[Solution Description] The nucleophilic substitution reaction involves a step where the nucleophile (ammonia) attacks the electrophile (ethyl bromide). This leads to the formation of an intermediate complex before the final product forms. Since we’re dealing with an $S_{\text{N}2}$ mechanism due to the primary nature of ethyl bromide, the initial nucleophilic attack by ammonia on ethyl bromide results in the formation of an unstable ammonium ion intermediate.

Key Concept: Classification

c) $CH_3NH_2$

[Solution Description] Primary amines contain one alkyl or aryl group attached to the nitrogen atom and are represented by the general formula $RNH_2.$ Among the given options, $CH_3NH_2$ has only one methyl $(CH_3)$ group attached to the nitrogen, making it a primary amine.

Key Concept: Mixed Amines

b) Mixed
[Solution Description] The given amine, $(CH_3)_2NCH_2CH_3,$ has two methyl groups $(CH_3)$ and one ethyl group $(CH_2CH_3).$ Since it contains different alkyl groups, this amine is classified as a mixed amine.

Key Concept: Basic Naming

a) Benzenediazonium chloride
[Solution Description] The compound $C_6H_5N_2^+Cl^-$ is named as benzenediazonium chloride. It consists of a benzene ring attached to a diazonium group $(N_2^+)$ and chloride as the counter ion.

Key Concept: Reaction Condition Analysis, Mechanistic Insight

c) Use of CuCl without heat application

[Solution Description] The conversion of benzenediazonium chloride to chlorobenzene involves the Gattermann reaction. In this reaction, copper chloride (CuCl) or cuprous chloride $(CuCl_2)$ acts as a catalyst to facilitate the replacement of the diazonium group by chlorine. It is crucial to maintain low temperatures during this reaction to stabilize the diazonium ion.

Key Concept: Hybridization and Geometry

c) Pyramidal
[Solution Description] Nitrogen in amines has $sp^3$ hybridization, which leads to a tetrahedral arrangement of electron pairs. However, due to the presence of a lone pair, the actual geometry of the molecule becomes pyramidal as three bonds are formed with atoms and one position is occupied by the lone pair.

Key Concept: Solubility and Boiling Point Correlation

c) Directly proportional
[Solution Description] The solubility of primary amines in water is directly proportional to their boiling points. Since primary amines engage in hydrogen bonding with water molecules, this interaction enhances both their solubility and boiling points due to stronger intermolecular attractions.

Key Concept: Mixed Amines

d) N,N-Dimethylbenzenamine
[Solution Description] The given compound has two methyl groups attached to the nitrogen atom of an aniline structure. According to IUPAC nomenclature rules, such compounds are named using ‘N,N-‘ to indicate the presence of identical substituents on the nitrogen. Thus, the correct IUPAC name is N,N-Dimethylbenzenamine.

Key Concept: Advanced Solubility Analysis, Real-World Application

a) Triethylamine
[Solution Description] Among amines, tertiary amines like triethylamine do not participate in intermolecular hydrogen bonding since they lack hydrogen atoms bonded to nitrogen. However, they may still exhibit moderate solubility in water due to their polarity without actively engaging in hydrogen bonding interactions.

Key Concept: Reaction Optimization, Comparative Analysis

c) Using slightly alkaline medium at low temperature
[Solution Description] The optimal condition for maximizing the yield involves maintaining a slightly basic environment to enhance electrophilic substitution. Phenol, being an activated aromatic compound towards electrophilic attack, couples efficiently with benzene diazonium salt under mild alkaline conditions. The presence of slight alkalinity prevents decomposition of the diazonium salt while enhancing the nucleophilicity of phenolate ion.

Key Concept: By-product Formation

c) FeCl$_2$
[Solution Description] The use of iron scrap and hydrochloric acid leads to the formation of FeCl$_2$ as a by-product. As the iron reacts with HCl, it generates this by-product while facilitating the reduction process.

Key Concept: Comparative Reactivity, Structural Analysis

a) Ethylamine
[Solution Description]
The rate of acylation reaction is determined by the availability of a lone pair on the nitrogen atom to attack the electrophile. Primary amines have more available lone pairs compared to secondary and tertiary amines because there are fewer steric hindrances. Tertiary amines generally do not participate in acylation due to lack of N-H bond necessary for the reaction to proceed via the formation of an amide. In this case, option (a), which is a primary amine, will undergo acylation most rapidly.

Key Concept: Reaction Conditions

c) Heating to 283 K
[Solution Description] The hydrolysis of diazonium salt to phenol requires heating to 283 K.

Key Concept: Advanced Synthesis, Reaction Pathway Analysis

c) Aniline

[Solution Description] The first step converts benzoic acid $(C_6H_5COOH)$ to benzoyl chloride $(C_6H_5COCl)$ using $SOCl_2.$ In the second step, treating benzoyl chloride with ammonia yields benzamide $(C_6H_5CONH_2).$ Finally, performing Hofmann’s bromamide reaction on benzamide leads to the formation of an amine, specifically aniline $(C_6H_5NH_2),$ by shortening the carbon chain and removing the carbonyl group.

Key Concept: Reaction Mechanism

b) Sandmeyer reaction
[Solution Description] Sandmeyer reaction is used to replace the diazonium group in arenediazonium salts with halides such as chloride, bromide using copper(I) halides.

Key Concept: Reaction Products

b) Isocyanide
[Solution Description] The carbylamine reaction involves the reaction of a primary amine with chloroform and ethanolic potassium hydroxide. The product formed in this reaction is an isocyanide, also known as carbylamine.

Key Concept: Structure-Effect Analysis

a) Primary $>$ Secondary $>$ Tertiary
[Solution Description]
In aqueous solutions, solvation and steric hindrance affect the basicity of amines. The basicity order is influenced such that primary amines are generally more basic than secondary, which are more basic than tertiary due to better solvation and less steric hindrance in primary amines compared to others.

Key Concept: Classification Basics

c) $(CH_3)_3N$

[Solution Description] Tertiary amines have three organic groups attached to the nitrogen atom and no hydrogen atoms. Among the options, $(CH_3)_3N$ represents a tertiary amine as all three positions are occupied by methyl groups.

Key Concept: Amino Group

d) Benadryl
[Solution Description] Benadryl is mentioned in the syllabus as containing a tertiary amino group. This indicates that it is the correct choice for this question.

Key Concept: Hydrogen Bonding

c) Hydrogen bonding
[Solution Description] Hydrogen bonding occurs between the nitrogen atom of the amine group and the hydrogen atoms of water, leading to the solubility of lower aliphatic amines in water.

Key Concept: Solubility and Molar Mass

d) Hexanamine

[Solution Description] The solubility of amines in water decreases as their molar mass increases. Among the given options, hexanamine has the highest molar mass, leading to the least solubility in water due to reduced hydrogen bond formation with water molecules.

Key Concept: Reaction Conditions

b) 273-278 K
[Solution Description] Aromatic amines form stable diazonium salts at low temperatures ranging from 273 K to 278 K when reacting with nitrous acid.

Key Concept: Reaction Condition

c) Heating
[Solution Description]
The reaction of potassium phthalimide with an alkyl halide requires heating to facilitate nucleophilic substitution. This involves the nucleophile from potassium phthalimide attacking the carbon atom in the alkyl halide, leading to the formation of an N-alkylphthalimide product.

Key Concept: Structural Isomerism

c) $(CH_3)_2-NH-CH_2CH_3$
[Solution Description] The molecular formula $C_4H_{11}N$ can have various structural isomers including primary, secondary, and tertiary amines. For it to be a secondary amine, two hydrogen atoms replaced by alkyl groups must link to one nitrogen atom $(R-NHR’).$ Possible options are:

$- CH_3-NH-C_3H_7$
$- (CH_3)_2-NH-CH_2CH_3$
$- CH_3-CH(NH(CH_3))-CH_2-CH_3$
$- NH(CH_2CH_2)(CH_2CH_2)$

Out of these, $(CH_3)_2-NH-CH_2CH_3$ is a valid structure for a secondary amine.

Key Concept: Basic Reaction

c) Iodobenzene
[Solution Description] Potassium iodide replaces the diazonium group in benzenediazonium chloride to form iodobenzene.

Key Concept: Ammonolysis Reaction

a) Ethylamine
[Solution Description]
The ammonolysis reaction of ethyl chloride involves replacing the chlorine atom with an amino group, resulting in the formation of ethylamine. The reaction can be represented as follows:

$C_2H_5Cl + NH_3 \rightarrow C_2H_5NH_2 + HCl$

In this process, the chlorine atom in ethyl chloride is substituted by the amine group from ammonia, forming a primary amine, which is ethylamine.

Key Concept: Coupling Reaction

c) p-aminoazobenzene
[Solution Description]
In a coupling reaction, benzenediazonium chloride reacts with aniline to form p-aminoazobenzene. This process retains the diazo group, forming an azo compound.

Key Concept: Tertiary Amine Naming

b) N,N-Dimethylmethanamine
[Solution Description] The compound $(CH_3)_3N$ consists of three methyl groups attached to the nitrogen, identifying it as a tertiary amine. Using IUPAC rules for naming such compounds, we add ‘N-‘ for each methyl substituent and end with ‘amine’. Therefore, the correct name is N,N-Dimethylmethanamine.

Key Concept: Reaction Specificity

c) Secondary amines
[Solution Description] While primary aliphatic and aromatic amines can form diazonium salts when reacting with nitrous acid, secondary and tertiary amines behave differently. Secondary and tertiary amines do not form diazonium salts upon reacting with nitrous acid due to their structural characteristics:
– Primary aliphatic amines: Form unstable diazonium salts
– Primary aromatic amines: Form stable diazonium salts
– Secondary amines: Do not form diazonium salts
– Tertiary amines: Do not form diazonium salts
Thus, secondary and tertiary amines specifically do not form diazonium salts under these conditions.

Key Concept: Electrophilic Substitution, Influence of Alkyl Groups

c) Para position
[Solution Description] For electrophilic substitution reactions in aniline derivatives like N,N-dimethylaniline, the directing influence of the amino group must be considered. In this case:

1. The dimethylamino group is highly activating and ortho/para-directing due to its +I and +R effects.
2. Under acidic conditions, protonation of the amino group decreases its activating effect but still directs substitution primarily to para positions over ortho, due to steric hindrance from bulky substituents.
3. The para position remains more accessible for incoming electrophiles.

Therefore, the bromination occurs predominantly at the para position relative to the dimethylamino group.

Key Concept: IUPAC Naming

c) 2-Propanamine
[Solution Description] The molecular formula $CH_3-CH(CH_3)-NH_2$ represents a primary amine where an $NH_2$ group is attached to the second carbon atom of propane. To determine the IUPAC name, we identify the longest carbon chain and replace the ‘e’ in propane with ‘amine’. Since $NH_2$ is on the second carbon, it is called 2-propanamine.

Key Concept: Reaction Mechanism

c) High temperature

[Solution Description] To form diazonium salts, a primary aromatic amine needs to react with nitrous acid $(HNO_2),$ which is typically generated in situ by mixing sodium nitrite $(NaNO_2)$ with hydrochloric acid $(HCl).$ The reaction should be performed at low temperatures (273-278 K) to ensure stability and proper formation of the diazonium salt. Thus, an acidic medium and controlled temperature are crucial.

Key Concept: Mechanistic Insight, Reaction Condition Analysis

a) It avoids formation of secondary and tertiary amines due to the stability of the intermediate.

[Solution Description] Gabriel phthalimide synthesis is particularly useful because it selectively produces primary amines without forming secondary or tertiary amines as by-products. The reaction involves the formation of a potassium salt of phthalimide using ethanolic KOH, which then reacts with an alkyl halide to form N-alkylphthalimide. This compound can be hydrolyzed to yield the primary amine and phthalic acid, without any possibility of forming secondary or tertiary amines since the nitrogen is bound in a cyclic structure until hydrolysis.

Key Concept: Reaction Conditions

d) With tertiary amines.
[Solution Description] Benzenesulphonyl chloride fails to react with tertiary amines. Unlike primary and secondary amines, tertiary amines do not possess a hydrogen atom attached to nitrogen that would facilitate the nucleophilic substitution reaction necessary for forming a sulphonamide.

Key Concept: Synthetic Uses

b) Hexamethylenediamine
[Solution Description] Amines serve as intermediates in the polymerization process, particularly those containing functional groups that react to form long polymer chains. An example often used in the production of nylon and other polyamides is hexamethylenediamine.

Key Concept: Basic Concept

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

Acetylation involves the introduction of an acetyl group $(CH_3CO-)$ to aniline, forming acetanilide. This process decreases the electron density on the nitrogen atom’s lone pair because it participates in resonance with the carbonyl group of the acetyl moiety. Consequently, the activating effect of the amino group is diminished, reducing the overall reactivity of aniline in electrophilic substitution reactions such as nitration and sulphonation.

Key Concept: Hydrogen Bonding Comparison

a) Primary amines
[Solution Description] Primary amines show the most extensive intermolecular hydrogen bonding because they have two hydrogen atoms attached to the nitrogen atom, allowing for more opportunities to engage in hydrogen bonding compared to secondary and tertiary amines, which have lesser hydrogen atoms available or none in the case of tertiary amines.

Key Concept: Protecting Groups

c) To control substitution
[Solution Description] The $-NH_2$ group is highly reactive due to its ability to donate electrons. During nitration, this can lead to polysubstitution. Acetylation of the -NH2 group forms acetanilide, decreasing its reactivity and controlling the extent of nitration, thus helping obtain mononitrated products efficiently.