Organic Chemistry - II
Organic Chemistry -II Synopsis
Synopsis
Alcohols
- Alcohols are hydroxyl derivatives of alkanes obtained by the replacement of one, two or three hydrogen atoms of alkanes by the corresponding number of –OH groups.
- The hydroxyl group is the functional group of alcohols.
- The general molecular formula of alcohols is CnH2n+1 OH.
Sources
- Ethanol is obtained from the fermentation of sugars, while methanol is obtained from the destructive distillation of wood.
- Cracking of petroleum is a source of ethane which is used for preparing ethanol.
- Preparation of Ethanol
- Laboratory preparation by hydrolysis of alkyl halides
- Industrial Method
- Hydration of Ethene
C2H5HSO4 + H2O →C2H5OH + H2SO4 - Fermentation of Carbohydrates
Properties of Alcohols
- Physical properties
- Inflammable volatile liquid.
- The boiling point increases with an increase in the molecular weight.
Example: Methanol = 64.5°C and ethanol = 78.3°C
- Soluble in water and organic solvents.
- Ethanol is lighter than water with a density of 0.79 cm−1 at 293 K.
- Chemical properties
- Combustion (burning):
C2H5OH + 3O2 → 2CO2 + 3H2O - Oxidation with K2Cr2O7
- Action with Sodium
2C2H5OH + 2Na →2C2H5ONa + H2
(Sodium ethoxide) - Action with Acetic Acid
C2H5OH + CH3COOH → CH3COOC2H5 + H2O
(Ethyl acetate)5. - Action with Sulphuric Acid
- Action with PCl3
3C2H5OH + PCl3 → 3C2H5Cl + H3PO3
Uses of Alcohol
- As a solvent for gums and resins
- Used in thermometers because of its low freezing point
- In alcoholic drinks
- In the manufacture of chemicals and synthetic products such as dyes, perfumes, antiseptics, preservatives etc.
Carboxylic Acids
- Carboxylic acids are organic compounds containing a carboxylic group (–COOH) attached to an alkyl group or to a hydrogen atom.
- Representation of carboxylic acids: R-COOH [R is either –H or alkyl]
- The functional group of carboxylic acids: –COOH [carboxylic]
- The acidic character in carboxylic acids is because of the presence of the replaceable hydrogen atom in the carboxylic group.
Occurrence
Acids occur in the free state in many fruits and as esters in several essential oils.
- Preparation of Acetic Acid
- By oxidation of ethyl alcohol
- By hydrolysis of ethyl acetate
Properties
Physical properties
- It is a colourless liquid with a pungent smell.
- Boiling point is 118°C, and melting point is 17°C.
- It is a hygroscopic liquid; specific gravity at 0°C is 1.08.
- Miscible in water, alcohol and ether in all proportions.
- Chemical properties
- It is a weak acid and turns blue litmus red.
2CH3COOH + Zn → (CH3COO)2Zn + H2 ↑ - Reaction with Alkalis
CH3COOH + NaOH → CH3COONa + H2O
CH3COOH + NH4OH → CH3COONH4 + H2O - Reaction with Carbonates
2CH3COOH + Na2CO3 →2CH3COONa + H2O + CO2
CH3COOH + NaHCO3→CH3COONa + H2O + CO2 - Reaction with Alcohols
- Reaction with PCl3
CH3COOH + PCl5 → CH3COCl + POCl3 + HCl - Reduction
CH3COOH + 4[H] →C2H5OH + H2O
Uses of Acetic Acid
- As a solvent for resins, cellulose etc.
- As a laboratory reagent
- As vinegar
- In medicines
- For coagulating rubber from latex
Preparation of Aldehydes and Ketones
- Oxidation of Alcohols
Aldehydes and ketones can be prepared by oxidation of primary and secondary alcohols.
- Dehydrogenation of Alcohols
On passing vapours of alcohol over metal catalysts like Cu at 573K, dehydrogenation takes place.
Primary alcohols give aldehydes whereas secondary alcohols yield ketones.
- From Hydrocarbons
By ozonolysis of Alkenes
Alkenes on ozonolysis followed by reaction with zinc dust and water yields aldehydes and ketones.
- By Hydration of Alkynes
Alkynes undergo addition reaction with water in the presence of got dil. H2SO4 and HgSO4 to give aldehydes or ketones.
Preparation for Aldehydes - Rosenmund Reduction
In this reaction, acyl chloride on hydrogenation in the presence of palladium catalyst and barium sulphate gives aldehydes.
- Stephen Reaction
Nitriles on reduction with stannous chloride in the presence of HCl give imine which on hydrolysis gives corresponding aldehyde.
An alternate method to reduce nitriles selectively is by diisobutylaluminium hydride to imines which on hydrolysis yields aldehydes.
Esters can also be reduced to aldehydes with DIBAL-H
- From Aromatic Hydrocarbons
Aromatic aldehydes can be prepared using the following methods.
- IBy Oxidation of Methylbenzene
Etard Reaction ( Use of Chromyl Chloride)
Chromyl chloride oxidises the methyl group to a chromium complex which on further gives corresponding benzaldehyde.
Use of Chromic oxide(CrO3)
Toluene when treated with chromic oxide in acetic anhydride gets converted into benzylidene diacetate which on hydrolysis with aqueous acid gives benzaldehyde.
- Side Chain Chlorination
Toluene on side chlorination gives benzal chloride which on hydrolysis gives benzaldehyde.
- Gatterman –Koch Reaction
Benzene or toluene on treatment with CO and HCl in the presence of AlCl3 or CuCl gives benzaldehyde or p-tolualdehyde.
Preparation for Ketones
- From Acid chlorides or Acyl chlorides
Acyl chloride on treatment with dialkylcadmium obtained by reaction of cadium chloride with Grignard reagent gives ketones.
Example:
- From Nitriles
Nitriles on treatment with Grignard reagent followed by hydrolysis yields a ketone.
- From Benzene or Substituted Benzenes
Benzene or substituted benzene on treatment with acid chloride in the presence of anhydrous AlCl3 gives the corresponding ketone and this reaction is known as Friedel-Crafts acylation reaction.
Physical Properties
Boiling Points
- The boiling points of aldehydes and ketones are higher than those of hydrocarbons and ethers of comparable molecular masses.
- This is because the carbonyl group in aldehydes and ketones is polar and hence show weak intermolecular association due to dipole-dipole interactions between the opposite ends of the
dipoles. - They have lower boiling points than those of alcohols of similar masses due to absence of intermolecular hydrogen bonding.
Solubility
- The lower members of aldehydes and ketones such as methanol, ethanol and propanone are miscible in water because they form hydrogen bond with water.
- The solubility decreases on increasing the length of alkyl chain.
- All aldehydes and ketones are fairly soluble in organic solvents like benzene, ether, methanol etc.
Chemical Reactions
- Nucleophilic Addition Reactions
Aldehydes and Ketones undergo nucleophilic addition reactions.
(i)Mechanism for Nucleophilic Addition Reactions
- A nucleophile attacks the electrophilic carbon atom of the polar carbonyl group perpendicularly to the sp3 hybridised orbitals of carbonyl carbon.
- The hybridisation changes from sp2 to sp3 and a tetrahedral alkoxide intermediate is formed.
- The intermediate grabs a proton from the reaction medium to give an electrically neutral product.
- The net result is addition of Nu- and H+ across the C-O double bond.
(ii)Reactivity
- Aldehydes are more reactive than ketones in nucleophilic reactions because of two reasons:
- Sterically, it is the presence of two relatively large groups in ketones that hinder the approach of nucleophile to carbonyl carbon than in aldehydes which have only one such substituent.
- Electronically, aldehydes are more reactive than ketones because the two alkyl groups in ketones decrease the electrophilicity of the carbonyl carbon more effectively than in aldehydes.
(iii)Important Examples of Nucleophilic Addition and Nucleophilic Addition- Elimination Reactions
(a)Addition of Hydrogen cyanide(HCN)
- On addition of HCN to aldehydes and ketones they yield cyanohydrins.
- Since the reaction is very slow with pure HCN, it is catalysed with the help of a base and the cyanide ion (CN-) generated as a strong nucleophile adds to carbonyl compounds to give cyanohydrins.
(b)Addition of Sodium Hydrogensulphite
- Sodium hydrogen sulphite when added to aldehydes and ketones yield addition products.
- For most aldehydes the equilibrium is on the right hand side and for most ketones it is on the left hand side due to steric factors.
(c)Addition of Grignard Reagents
Grignard reagents on reacting with aldehydes and ketones yield alcohols.
For example:
- Methanal produces primary alcohol with Grignard reagent.
- Aldehydes produce secondary alcohols with Grignard reagent.
- Ketones produce tertiary alcohols with Grignard reagent.
(d)Addition of Alcohols
- Aldehydes on treatment with one equivalent of monohydric alcohol in the presence of dry HCl give hemiacetal which on further treatment with one more molecule of alcohol gives acetal.
- Ketones also react with ethylene glycols under similar conditions to give ethylene glycol ketals.
- The role of dry HCl is to protonate the oxygen of carbonyl compounds and thereby increasing the electrophilicity of the carbonyl carbon towards nucleophilic addition of ethylene glycol.
(e)Addition of Ammonia and its Derivatives
- Ammonia and its derivative add to the carbonyl group of an aldehydes and ketones.
- The reaction is reversible and acid catalysed and favours the product formation due to the rapid dehydration of the intermediate to form >C=N-Z.
- Reduction
- Reduction to Alcohols
Aldehydes and ketones get reduced to primary and secondary alcohols by NaBH4 or LiAlH4.
- Reduction to Hydrocarbons
Aldehydes and ketones reduce to –CH2 group on treatment with zinc-amalgam and conc. HCl[Clemmenson reduction] or with hydrazine which on heating with sodium or potassium hydroxide in ethylene glycol[Wolff-Kishner reduction].
Oxidation
- Aldehydes get oxidised to carboxylic acids with common oxidising agents like nitric acid, potassium permanganate, potassium dichromate etc.
- Ketones undergo oxidation with strong oxidising agents and elevated temperatures. The reaction involves carbon-carbon bond cleavage to give a mixture of carboxylic acids with lesser number of carbon atoms than the parent ketones.
Carbohydrates
Carbohydrates and Their Classification
Carbohydrates form a very large group of naturally occurring organic compounds which play a vital role in daily life. They are produced in plants by the process of photosynthesis. The most common carbohydrates are glucose, fructose, sucrose, starch, cellulose etc. Carbohydrates may be defined as optically active polyhydroxy aldehydes or ketones or the compounds which produce such units on hydrolysis.
The most common sugar, used in our homes is named as sucrose whereas the sugar present in milk is known as lactose. Depending upon their behaviour on hydrolysis carbohydrates are classified into three groups.
Classification of Carbohydrates