1. What are carboxylic acids?
Carboxylic acids are the compounds containing the carboxyl group in their molecules.
C=O
|
OH
The carboxyl group is made up of carbonyl, -C=O and hodroxyl, -OH group.
2.. What is the common system of nomenclature of carboxylic acids?
The common names of carboxylic acids are based on their source of origin. Formic acid was first obtained from red ants (Latin formica means red ants) and it was named from that. Acetic acid was obtained from vinegar (Latin aceum means vinegar) and so got that name.
In the common system the position of substituents is indicated by the Greek letters α, β, γ, ō.
α, β, γ, ō Carbon atoms
The carbon atom next to the carboxyl carbon is assigned the letter α. The carbon next to α-carbon is the β-carbon. The carbon next to β-carbon is the γ-carbon. The carbon next to γ-carbon is the ō Carbon.
ō-γ-β-α carbons
C-C-C-C-COOH
3. What is the IUPAC system of nomenclature of carboxylic acids?
According to IUPAC system, the name of the monocarboxylic acid is derived by changing the final 'e' from the name of the corresponding hydrocarbon with 'oic' and adding the word acid.
Formic acid - Methanoic acid
Acetic acid - Ethanoic acid
n-Butyric acid - Butanoic acid
Isobutyric acid - 2-Mehtylpropanoic acid
Dicarboxylic acids
Oxalic acid - Ethanedioic acid
Malonic acid - Propanedioic acid
The position of substituents is indicated by the following rules.
1. The longest chain of carbon atoms containing the carboxylic group(-COOH) is selected.
2. The numbering of carbons starts from the carboxylic acid group and the carbon of carboxyl group is given number 1.
3. The position of the substituents is indicated by the number of carbon atom to which they are attached.
4. What is the IUPAC system of nomenclature of carboxylic acids if more than two acid groups are present?
If an unbrached chain is directly bonded to more than two like functional groups, the organic compound is named as a derivative of parent alkane which does not include the carbon atoms of the functional groups. These are named by the use of suffix such as tricarboxylic acid
For example: Pentane-1,3,5-tricarboxylic acid
But if three COOH groups are not directly linked to the unbranched chain (say one COOH is linked to an alkyl group on the branched chain), the two like groups are considered in the parent chain and are named by using the suffix di before the name of the functional group. The third group (part of the side chain) is considered as a substituent group.
Example: (3-Carboxymethyl) heptane -1,7-dioic acid
5. What is the IUPAC system of nomenclature of aromatic carboxylic acids?
The simplest aromatic carboxylic acid is benzoic acid.
The IUPAC names of substituted aromatic carboxylic acids are derived by prefixing the name of the substituent to the name of the parent acid i.e., benzoic acid and the position is indicated by an Arabic numeral with the carbon atom carrying the –COOH group being numbered as 1.
6. What is the IUPAC system of nomenclature of dicarboxylic acids?
These are named as alkanedioic acids.
Dicarboxylic acids
Common name – IUPAC name
Oxalic acid - Ethanedioic acid
Malonic acid - Propanedioic acid
7. Explain the methods of preparation of carboxylic acids.
a. From oxidation of primary alcohols
Primary alcohols are oxidized with potassium permanganate and potassium dichromate to aldehydes which on further oxidation give carboxylic acids.
b. from oxidation of aldehydes and ketones
Aldehydes are easily oxidized to carboxylic acids with mild oxidizing agents like Tollen;s reagent.
c. from hydrolysis of nitriles(cyanides)
The nitriles are hydrolysed in dilute aqueous acidic or alkaline medium.
The alkyl cyanides used for the purpose are prepared from corresponding alkyl halides or alcohols.
The acid produced contains one more carbon atom than the original alkyl halide or alcohol. Thus, this method is a useful method for the preparation of carboxylic acids containing one more carbon atom than the starting alkyl halide or alcohol.
d. from Grignard reagents
The reaction is carried out by bubbling CO2 through the etheral solution of suitable Grignard reagent.
e. by hydrolysis of esters
Hydrolysis of esters with mineral acids or alkalies gives carboxylic acids
f. carboxylation of alkenes
Heating alkenes with CO and steam under pressure with phosporic acid at 673 K. This reaction is called Koch reaction.
g. from trihalogen derivatives of hydrocarbons
Hydrolysis of 1,1,1,-trihalogen derivatives of alkanes with acqueous KOH.
h. preparation of aromatic acids from alkyl benzenes
the alkyl side chain of benzene ring can be easily oxidized to carboxylic group with alkaline KMnO4, chromic anhydride or conc. HNO3.
8. Explain the physical properties of carboxylic acids.
a. Physical state and smell
The first three members are colourless liquids and have pungent smell. The next six members are oily liquids with a faint unpleasant odour.
Still higher acids are colourless waxy solids.
Benzoic acids and its homologues are colourless solids.
b. Boiling points
They have higher boiling points than the corresponding alcohols of comparable molecular masses.
Carboxylic acids have higher boiling points due to the presence of intramolecular hydrogen bonding. Due to the hydrogen bonding, carboxylic acids exist as dimers.
c. Melting point
In the case of first ten carboxylic acids, the melting points of acids containing even number of carbon atoms is higher than the next lower and higher member containing odd number of carbon atoms.
The melting and boiling points of aromatic acids are usually higher than those of aliphatic acids of comparable molecular masses.
d. Solubility in water
The first four members of aliphatic carboxylic acids are very soluble in water. The solubility in water decreases gradually with rise in molecular mass. All are soluble in alcohol or ether.
Benzoic acid is sparingly soluble in cold water but is soluble in hot water, alcohol and ether.
9. Chemical properties or reactions of carboxylic acids
1. Acidic character
a. How do you express strength of carboxylic acids?
The strength of acids can be expressed in terms of dissociation constant Ka or Ph number of PKa number which is pKa = -log Ka
b. what is effect of substituents on strength of carboxylic acids?
i.. Effect of electron releasing substituents,
Electron releasing substituents: Alkyl is an electron releasing group. If the H atom of formic acid (HCOOH) is replaced by CH3 group to form acetic acid (CH3COOH) the alkyl group will tend to increase the electron density on the oxygen atom of the O-H bond. This increase will make removal H+ ion difficult in comparison to formic acid.
Acetic acid is a weaker acid in comparison to formic acid.
The electron release effect is called +I effect. As +I effect increases, acidic strength will go down. As more alkyl groups are there +I effect increases
CH3 is less than C2H5 is less than (CH3)2CH
Therefore acidic property is stronger or more for CH3COOH.
Acidic strength is in the following order
acidic strength of HCOOH>CH3COOH>CH3CH2COOH>(CH3)2CHCOOH
ii. Effect of electron withdrawing substituents
Electron withdrawing substituents: Substituents like halogens tend to withdraw the electron charge. Halogens are electron attracting atoms(-I inductive effect). They withdraw the electrons from the carbon to which they are attached and this effect is transmitted throughout the chain. The increases positive charge on O atom in the O-H bond and dissociation of H+ ion or proton takes place more easily.
Hence chloroacetic acid is stronger acid than acetic acid.
c. compare relative acidic strength of carboxylic acids and alcohols.
d. compare relative acidic strength of carboxylic acids and phenols
e. What are the reactions that show acidic charater of carboxylic acids?
(i). Action with blue litmus
all carboxylic acids turn blue litmus red.
(ii). Reaction with metals: liberation of hydrogen
Carboxylic acids react with active metals such as Na, K, Ca, Mg, Zn, etc., to form their salts with the liberation of hydrogen.
(iii). Action with alkalies: formation of salts
Carboxylic acids neutralize alkalies forming salts and water.
(iv). Action with carbonates and bicarbonates: evolving carbon dioxide
Carboxylic acids decompose carbonates and bicarbonates evolving carbon dioxide with brisk effervescence.
2. Formation of acid chlorides
3. Formation of esters
When carboxylic acids are heated with alcohols in the presence of concentrated sulphuric acid, esters are formed.
a. Mechanism of esterification
4. Formation of amides
Carboxylic acids react with ammonia to form ammonium salts.
Acetic acid + Ammonia gives Ammonium acetate.
Ammonium acetate on heating gives acetamide plus water.
5.Formation of acid anhydrides
Carboxylic acids on heating in the presence of a strong dehydrating agent such as phosphorous pentoxide form acid anhydrides.
6. Decarboxylation of carboxylic acids
- electrolytic decarboxylation
Salts of carboxylic acids get decarboxylated – lose carbon dioxide in some reactions.
a. Sodium or potassium salts of carboxylic acids on heating with soda lime give alkanes.
b. Electrolysis of acqueous solutions of sodium or potassium salts of carboxylic acids gives alkanes due to decarboxylation.
c. When calcium salts of monocarboxylic acids (fatty acids) are heated aldehydes or ketones are formed.
7a. Partial reduction to alcohols
a. Partial reduction: Carboxylic acids on reduction with lithium aluminium hydride or with hydrogen in the presence of copper chromite are reduced to alcohols.
b. Complete reduction to alkanes: Carboxylic acids on reduction with HI and red P give alkanes.
8. Action of bromide on silver salt of the acid – Hunsdiecker reaction
The silver saltsof the carboxylic acid on treatment with Br2 in the presence of CCl4 give alkyl halides having one carbon atom less than the parent acid.
9. Halogenation
Carboxylic acids react with chlorine on bromine in the presence of a small amount of phosphorus to give halogenated compounds. The reaction is called Hell Volhard-Zelinksy reaction.
10 Ring substitution in aromatic acids – bromination, sulphonation, nitration
In substitution reactions with aromatic carboxylic acids, carboxyl group is an electron withdrawing group and therefore it favours meta position for the substituent.
Hence benzoinc acid + bromine gives 3-Bromobenzoic acid
Showing posts with label Conceptual questions. Show all posts
Showing posts with label Conceptual questions. Show all posts
Thursday, January 8, 2009
Organic Nitrogen Compounds - Amines - Conceptual Questions - Answers
1. What are amines?
Amines are regarded as derivatives of ammonia in which one, two or all three hydrogen atoms are replaced by alkyl or aryl group.
2. Amines are derivatives of ammonia .
3. What are quaternary ammonium compounds?
These are compounds which are regarded as derivatives of ammonium salts in which all the four H-atoms are replaced by alkyl or aryl groups.
4. What are the various classifications of amines?
In one classification, Amines are classified into two categories: Aliphatic amines and aromatic amines.
1. Aliphatic amines: Amines in which the nitrogen atom is directly bonded to one or more alkyl groups.
2. Aromatic amines: Amines which have aromatic groups in the compounds. They are further classified as aryl amines and arylalkyl amines or side chain substituted amines.
a. Aryl amines: In these amines, the nitrogen atom is directly bonded to one or more aromatic rings or aryl groups.
b. Arylalkyl amines: In these amines, the nitrogen atom is bonded to the side chain of the aromatic ring.
In second type of classification, amines are classified as simple and mixed amines. This classification is applicable to secondary and tertiary amines which have two or three alkyl or aryl groups.
If the two or three alkyl or aryl groups are the same, it is a simple amine. If the alkyl groups are different, it is a mixed amine.
5. What is the common system of nomenclature of aliphatic amines?
In the common system, amines are called alkylamines. The suffix amines is added to the name of the corresponding alkyl group.
Ex: Methylamine, Ethylamine, Propylamine
Second common system
In the second system of the common system, primary amines are named as the amino derivatives of the corresponding hydrocarbons and are names as aminoalkanes. The position of the amino group is indicated by Arabic numeral. The numbering of the primary chain is to be done in such a way that the carbon atom containing the amino group gets the lowest possible number.
Secondary and tertiary amines are named as nitrogen substituted primary amine.
Example: N-Methylaminoethane
Primary amine is aminoethane and N-Methyl is added to it. N- indicates that the methyl group is attached to the nitrogen atom.
6. What is the IUPAC system of nomenclature of aliphatic amines?
The aliphatic amines are called alkamines. The letter ‘e’ in the alkane is replaced by suffix amine.
Ex: methanamine, ethanamine
The position of the amino group is indicated by Arabic numeral. The numbering of the primary chain is to be done in such a way that the carbon atom containing the amino group gets the lowest possible number.
Ex: 1-propanamine, 2-propanamine
7. What is the common system of nomenclature of aromatic amines?
Aromatic amines are called aryl amines. Suffix amine is added to the aryl group.
8. What is the IUPAC system of nomenclature of aromatic amines?
The simplest aromatic amine C6H5NH2 is called benzanamine.
Other aromatic amines are named as derivatives of benzenamine and positions of other groups are indicated by numbers
Ex: Benzenamine
N-Methylbenzenamine, N,N-Dimethylbenzenamine, 2-Methylbenzenamine, 3-Methylbenzenamine
In IUPAC system, benzenamine may also be written as amino benzene.
9. Explain chain isomerism in amines.
Aliphatic amines containing four or more carbon atoms show chain isomerism as branched and straight chain alkyl groups can be attached to the nitrogen atom.
Example: Butan-1-amine, 2-Methylpropan-1-amine, and 2 Methylpropan-2-amine are chain isomers.
10. Explain metamerism in amines.
In amines metamerism, a type of isomerism in which different alkyl groups are attached to the Nitrogen atom of the amino group exists(molecular formula for isomers is same).
Example: Diethylamine, Methyl-n-propylamine, and Isopropylmethylamine are metamers.
11. Explain position isomerism in amines.
The amino group can be bonded to different carbon atoms of the alkyl group.
12. Explain functional isomerism in amines.
Primary, secondary and tertiary amines having the same molecular formula are isomers. This isomerism is functional isomerism because different alkyl groups (functional groups) are present in the molecules and give rise to isomerism
13. Explain various methods of preparing amines.
1. From alkyl halides –
By ammonolysis (Hoffmann’s method): When an aqueous or alcoholic solution of ammonia is heated with an alkyl halide at 373 K in a sealed tube, a mixture of three amines (primary, secondary and tertiary) is obtained. It is very difficult to separate the mixture.
By Gabriel’s phthalimide synthesis
In this synthesis, phthalimide is treated with alcoholic KOH to give potassium phthalimide.
Potassium phthalimide is treated with alky halide to form N-alkyl phthalimide.
The hydrolysis of N-alkyl phthalimide with 20% HCl under pressure or refluxing with NaOH gives primary amine.
This method can be used for preparing only primary amines.
2. Reduction of nitro compounds – by hydrogen, by active metals, and by LiAlH4
3. Reduction of nitriles (cyanides) and isonitriles (isocyanides)
Nitriles can be reduced to corresponding amines using H2/Raney Li or Pt, LiAlH4 or Na, C2H5OH
When sodium and alcohol are used, the reaction is called Mendius reaction
Isonitriles can be reduced to secondary amines using H2/Raney Li or Pt, LiAlH4 or Na, C2H5OH
4. From Amides – by reduction of amides, by Hofmann degradation method
Amides on treatment with Br2 and KOH give primary amines. The amine formed contains one carbon atom less than the parent amide.
5. From Oximes – by reduction
Oximes are obtained from aldehydes and ketones by reaction with hydroxylamine. The oximes of aldehydes or ketones can be reduced to primary amines with either Na/CH2H5OH or LiAlH4.
6. Reductive amination of aldehydes and ketones
Reaction between aldehydes or ketones and ammonia results in the formation of imines. Imines are reduced to primary amines with H2, Ni.
7. From alcohols – Sabatier and Mailhe method
8. Preparation of aniline from nitrobenzene
Amines are regarded as derivatives of ammonia in which one, two or all three hydrogen atoms are replaced by alkyl or aryl group.
2. Amines are derivatives of ammonia .
3. What are quaternary ammonium compounds?
These are compounds which are regarded as derivatives of ammonium salts in which all the four H-atoms are replaced by alkyl or aryl groups.
4. What are the various classifications of amines?
In one classification, Amines are classified into two categories: Aliphatic amines and aromatic amines.
1. Aliphatic amines: Amines in which the nitrogen atom is directly bonded to one or more alkyl groups.
2. Aromatic amines: Amines which have aromatic groups in the compounds. They are further classified as aryl amines and arylalkyl amines or side chain substituted amines.
a. Aryl amines: In these amines, the nitrogen atom is directly bonded to one or more aromatic rings or aryl groups.
b. Arylalkyl amines: In these amines, the nitrogen atom is bonded to the side chain of the aromatic ring.
In second type of classification, amines are classified as simple and mixed amines. This classification is applicable to secondary and tertiary amines which have two or three alkyl or aryl groups.
If the two or three alkyl or aryl groups are the same, it is a simple amine. If the alkyl groups are different, it is a mixed amine.
5. What is the common system of nomenclature of aliphatic amines?
In the common system, amines are called alkylamines. The suffix amines is added to the name of the corresponding alkyl group.
Ex: Methylamine, Ethylamine, Propylamine
Second common system
In the second system of the common system, primary amines are named as the amino derivatives of the corresponding hydrocarbons and are names as aminoalkanes. The position of the amino group is indicated by Arabic numeral. The numbering of the primary chain is to be done in such a way that the carbon atom containing the amino group gets the lowest possible number.
Secondary and tertiary amines are named as nitrogen substituted primary amine.
Example: N-Methylaminoethane
Primary amine is aminoethane and N-Methyl is added to it. N- indicates that the methyl group is attached to the nitrogen atom.
6. What is the IUPAC system of nomenclature of aliphatic amines?
The aliphatic amines are called alkamines. The letter ‘e’ in the alkane is replaced by suffix amine.
Ex: methanamine, ethanamine
The position of the amino group is indicated by Arabic numeral. The numbering of the primary chain is to be done in such a way that the carbon atom containing the amino group gets the lowest possible number.
Ex: 1-propanamine, 2-propanamine
7. What is the common system of nomenclature of aromatic amines?
Aromatic amines are called aryl amines. Suffix amine is added to the aryl group.
8. What is the IUPAC system of nomenclature of aromatic amines?
The simplest aromatic amine C6H5NH2 is called benzanamine.
Other aromatic amines are named as derivatives of benzenamine and positions of other groups are indicated by numbers
Ex: Benzenamine
N-Methylbenzenamine, N,N-Dimethylbenzenamine, 2-Methylbenzenamine, 3-Methylbenzenamine
In IUPAC system, benzenamine may also be written as amino benzene.
9. Explain chain isomerism in amines.
Aliphatic amines containing four or more carbon atoms show chain isomerism as branched and straight chain alkyl groups can be attached to the nitrogen atom.
Example: Butan-1-amine, 2-Methylpropan-1-amine, and 2 Methylpropan-2-amine are chain isomers.
10. Explain metamerism in amines.
In amines metamerism, a type of isomerism in which different alkyl groups are attached to the Nitrogen atom of the amino group exists(molecular formula for isomers is same).
Example: Diethylamine, Methyl-n-propylamine, and Isopropylmethylamine are metamers.
11. Explain position isomerism in amines.
The amino group can be bonded to different carbon atoms of the alkyl group.
12. Explain functional isomerism in amines.
Primary, secondary and tertiary amines having the same molecular formula are isomers. This isomerism is functional isomerism because different alkyl groups (functional groups) are present in the molecules and give rise to isomerism
13. Explain various methods of preparing amines.
1. From alkyl halides –
By ammonolysis (Hoffmann’s method): When an aqueous or alcoholic solution of ammonia is heated with an alkyl halide at 373 K in a sealed tube, a mixture of three amines (primary, secondary and tertiary) is obtained. It is very difficult to separate the mixture.
By Gabriel’s phthalimide synthesis
In this synthesis, phthalimide is treated with alcoholic KOH to give potassium phthalimide.
Potassium phthalimide is treated with alky halide to form N-alkyl phthalimide.
The hydrolysis of N-alkyl phthalimide with 20% HCl under pressure or refluxing with NaOH gives primary amine.
This method can be used for preparing only primary amines.
2. Reduction of nitro compounds – by hydrogen, by active metals, and by LiAlH4
3. Reduction of nitriles (cyanides) and isonitriles (isocyanides)
Nitriles can be reduced to corresponding amines using H2/Raney Li or Pt, LiAlH4 or Na, C2H5OH
When sodium and alcohol are used, the reaction is called Mendius reaction
Isonitriles can be reduced to secondary amines using H2/Raney Li or Pt, LiAlH4 or Na, C2H5OH
4. From Amides – by reduction of amides, by Hofmann degradation method
Amides on treatment with Br2 and KOH give primary amines. The amine formed contains one carbon atom less than the parent amide.
5. From Oximes – by reduction
Oximes are obtained from aldehydes and ketones by reaction with hydroxylamine. The oximes of aldehydes or ketones can be reduced to primary amines with either Na/CH2H5OH or LiAlH4.
6. Reductive amination of aldehydes and ketones
Reaction between aldehydes or ketones and ammonia results in the formation of imines. Imines are reduced to primary amines with H2, Ni.
7. From alcohols – Sabatier and Mailhe method
8. Preparation of aniline from nitrobenzene
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