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ORGANIC CHEMISTRY-II
Prof. Om Prakash Department of Chemistry College of basic Sciences and Humanities G.B. Pant University of Agriculture & Technology Pantnager
Published by : Uttarakhand Open University, Haldwani, Nainital- 263139
Title : ISBN No. : Copyright : Edition :
Organic Chemistry- II Uttarakhand Open University 2018
CONTENTS
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1.1 OBJECTIVES
Objectives of this unit are to study the alcohols, their structures, nomenclature, and classification on the basis of number of –OH groups present like monohydric alcohol, dihydric and polyhydric alcohols. Classification on the basis of nature of carbon attached with –OH group like primary, secondary and tertiary alcohols. This unit also aims on methods of preparation of alcohols with their physical and chemical properties, acidic and basic characters. Chemical reactions of alcohols like Acid- catalyseddehydration etc, Study on chemical properties of dihydric and polyhydric alcohos have also been aimed in this unit
1.2 INTRODUCTION
Alcohols are organic compounds in which one or more hydrogen atoms from hydrocarbon have been replaced by hydroxyl (-OH) group. They are some of the most common and useful compounds in nature, in industry, and around the house. The general formula for a simple acyclic alcohol is CnH2n+1OH, where n=1, 2, 3, etc. The saturated carbon chain is often designated by the symbol R, so that ROH can represent any alcohol in the homologous series. Alcohols can be viewed as organic analogues of water in which one hydrogen atom is replaced by an alkyl group. The simplest and most commonly used alcohols are methanol and ethanol. They occur widely in nature and have many industrial and pharmaceutical applications.
Aromatic compounds, which contain a hydroxy group on a side chain, behave like alcohols are called aromatic alcohol. In these alcohols, the —OH group is attached to a sp^3 hybridised carbon atom next to an aromatic ring.
cyclopropanol cyclohexanol (^) isobutanol
methanol (^) ethanol
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In some alcohols, the —OH group is attached to a sp^3 hybridised carbon next to the carbon-carbon double bond that is to an allylic carbon are known as allylic alcohols. In some alcohols —OH group bonded to a carbon-carbon double bond i.e., to a vinylic carbon or to an aryl carbon. These alcohols are also known as vinylic alcohols. Allylic and benzylic alcohols may be primary, secondary or tertiary in nature.
1.3 CLASSIFICATION OF ALCOHOLS
Alcohols are classified into following types on the basis of number of –OH groups present in the molecule and nature of carbon attached with –OH group as follow:
(a) Monohydric Alcohols: These compounds contain only one –OH group.
(b) Dihydric Alcohols: These contain two –OH groups.
(c) Trihydric Alcohols: These contain three –OH groups.
benzyl alcohol (^2) - phenyl ethanol 3 - phenyl propanol
vinylic alcohol (^) allylic alcohol
phenol
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C H
C
CH 3
OH
If we replace hydrogen with a –OH group we get the following groups for three alcohols:
primary alcohol (^) secondary alcohol (^) tertiary alcohol
1.4 NOMENCLATURE OF ALCOHOLS
According to the IUPAC system of nomenclature, alcohols are called alkanols. They are named as the derivatives of the corresponding alkane in which the - e of the alkane is replaced by -ol****. The IUPAC have come up with a set of rules that are used to name any alcohol regardless of its complexity. These rules are summarized as follows:
Step 1. Name the longest continuous chain to which the hydroxyl (—OH) group is attached. Count the number of carbon atoms and identify the corresponding alkane. The name for this chain is obtained by dropping the final -e from the name of the hydrocarbon parent name and adding the ending -ol.
Step 2. Number the longest chain to give the lowest possible number to the carbon bearing the hydroxyl group.
Step 3. Locate the position of the hydroxyl group by the number of the carbon to which it is attached. Step 4. Number the any other substituents according to their position on the chain.
primary alcohol (^) secondary alcohol (^) secondary alcohol tertiary alcohol
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Step 5. Combine the name and location for other groups, the hydroxyl group location, and the longest chain into the final name.
Step 6. If there are more than one –OH group do not remove the –e from the suffix, but add a di- or tri- prefix to the –ol suffix.
Step 7. Identify and locate the other branches on the chain so that they are named alphabetically and their carbon number is hyphenated onto the front of the name.
viz; Alcohols Common name IUPAC name CH 3 OH Methyl alcohol Methanol
CH 3 CH 2 OH Ethyl alcohol Ethanol
CH 3 CH 2 CH 2 OH n-Propyl alcohol 1-Propanol
CH 3 CHOHCH 3 Isopropyl alcohol 2-Propanol
CH 3 (CH 2 ) 2 CH 2 OH n-Butyl alcohol 1-Butanol
CH 3 (CH 2 ) 3 CH 2 OH n-Pentyl alcohol 1-Pentanol
Other examples:
2 - butanol
(^5) methyl 2 hexanol
cyclopentanol
1 ,^2 ,^ 3- trihydroxy propane
(^3) - cyclopentyl - - (^1) propanol
1
2 (^1 2 3 )
(^3) -- amino 2-butanol
(^3) - butene - 2 -ol
3 - meyhylcyclohexanol
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2. Reduction of carbonyl compounds: Carbonyl compounds (which contain –C–O group) such as aldehydes, ketones, carboxylic acids and esters can be reduced to alcohols. Aldehydes give primary alcohols while ketones yield secondary alcohols, either by catalytic hydrogenation or by use of chemical reducing agents like lithium aluminum hydride, LiAlH4. Carboxylic acids and esters also give primary alcohols on reduction with hydride reagents such as LiAIH4 and sodium borohydride
(NaBH 4 ). NaBH 4 does not reduce carbon-carbon double bonds, not even those conjugated with carbonyl groups, and in thus useful for the reduction of such unsaturated carbonyl compounds to unsaturated alcohols.
In the above reactions it is observed that only the carbonyl group is reduced and the other functional groups remain unaffected. Highly selective behaviour of
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NaBH 4 makes it the preferred reagent for the reduction of carbonyl groups in sensitive polyfunctional group containing compounds.
3. From hydration of alkenes: double bond to give alcohols. This is an electrophilic addition of H Alcohols can be prepared by adding water to an alkene in the presence of a strong acid such as co. H 2 SO product of the reaction is often a highly substituted 2
RCH=CH 2 + H 2 SO
CH 2 =CH 2 + H 2 SO 4
Ease of preparation is tert. > same sequence.
4. Oxidation of organoboranes: in THF solution, an organoborane is obtained. Hydroboration followed by oxidation will produce an alcohol. Since BH three times to give trialkylborane. (H 2 O 2 ) in the presence of aqueous sodium hydroxide. addition of water across th the reaction is regioselective producing the least substituted alcohol.
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makes it the preferred reagent for the reduction of carbonyl groups in sensitive polyfunctional group containing compounds.
dration of alkenes: Hydration i.e.s addition of H+^ and OH– double bond to give alcohols. This is an electrophilic addition of H 2 O to the alkene. Alcohols can be prepared by adding water to an alkene in the presence of a strong SO4. Because these reactions follow Markovnikov's rule, the product of the reaction is often a highly substituted 2º^ or 3º^ alcohol.
SO 4 → RCH-CH 3 RCHOHCH 3
4 →^ CH 3 -CH 2 HSO 4 CH 3 CH 2 OH
ation is tert. > sec. > prim alcohol; ease of dehydration follows
Oxidation of organoboranes: When an alkene reacts with BH 3 (a boron hydride) in THF solution, an organoborane is obtained. Hydroboration followed by oxidation n alcohol. Since BH 3 has three hydrogens, above addition can occur three times to give trialkylborane. This is oxidised to alcohol by hydrogen peroxide in the presence of aqueous sodium hydroxide. The overall reaction is addition of water across the double bond opposite to that of Markovnikov’s rule and the reaction is regioselective producing the least substituted alcohol.
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makes it the preferred reagent for the reduction of carbonyl groups in sensitive polyfunctional group containing compounds.
across a C=C O to the alkene. Alcohols can be prepared by adding water to an alkene in the presence of a strong Because these reactions follow Markovnikov's rule, the
sec. > prim alcohol; ease of dehydration follows
(a boron hydride) in THF solution, an organoborane is obtained. Hydroboration followed by oxidation has three hydrogens, above addition can occur This is oxidised to alcohol by hydrogen peroxide The overall reaction is e double bond opposite to that of Markovnikov’s rule and
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(b) By reaction with esters: substituents on the hydroxyl
(c) By reaction with epoxides: alcohols containing two or more carbon atoms.
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(b) By reaction with esters: Produces tertiary alcohols in which two of the substituents on the hydroxyl- bearing carbon are derived from the Grignard reagent.
(c) By reaction with epoxides: Grignard reagents react with epoxide to yield primary alcohols containing two or more carbon atoms.
MgBr NH 4 + C OH
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Produces tertiary alcohols in which two of the bearing carbon are derived from the Grignard reagent.
react with epoxide to yield primary
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5. Fermentation: Ethanol is prepared on a large scale using fermentation process. It involves breaking down large molecules into simpler ones using enzymes. Usually, yeast is added as a source of enzymes. Yeast converts the reactant glucose or fructose into ethanol and carbon dioxide in presence of zymase enzyme.
1 .6 ACIDIC NATURE OF ALCOHOLS
Alcohols can act as Brönsted acids as well as Lewis base due to donation of proton and presence of unpaired electron on oxygen respectively. Alcohols are very weak acids because the alkyl group pushes electrons towards the —OH group, so that the oxygen does not strongly attract the electrons in the —OH bond. Furthermore once a RO-^ ion is formed, it cannot be stabilized by the delocalization of the charge. Thus alcohols react only to a very slight extent with alkali, but will react with very electropositive metals under anhydrous conditions to give alkoxide with the general formula RO-^ M+.
Example: Reaction of ethanol with sodium
2CH 3 CH 2 OH + 2Na 2CH 3 CH 2 O-^ Na+^ + H 2
Addition of water will regenerate the alcohol readily.
CH 3 CH 2 O-Na+^ + H 2 O CH 3 CH 2 OH + NaOH
The reaction is much slower than the reaction of water with sodium. Alcohols tend to be slightly less acidic (pKa = 15) compared to water (pKa = 14). The higher the pKa value the lower is the acid strength. The reaction of alcohol with sodium can be used to deposite the excess sodium in the laboratory. Even alcohols are neutral to litmus and do not reacts with alkali like NaOH but contain active hydrogen atom so reacts with Na or K metal.
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Insoluble in water
O H H O H 3 C H O H H
H
O H
solubility. That is why alcohols are much more soluble in water than their corresponding alkanes, aromatic hydrocarbons, alkyl halides or aryl halides. Amongst isomeric alcohols, the solubility increases with branching.
(iv)The B.P. and M.P. will also increase with carbon chain length. The longer the alcohols carbon chain, the better the chance that the alcohol will be a solid at room temperature. Alcohols show higher boiling points than alkane and ethers of similar mass due to hydrogen bonding. Since there is not any possibility of hydrogen bonding in ether, the forces between the ether molecules are much weaker and can be much more easily vaporized.
Comparison of boiling points among isomeric alcohols
(v) The viscosity of small alcohols is much higher than the viscosity of alkanes.
(vi) Generally alcohols are lighter than water, i.e., less dense than water. Density of alcohols increases with molecular mass.
R O H .....^ O^ H
R ..... O H
R
1 _^ butanol 2 _ (^) methyl 1 _ (^) propanol 2 _methyl 2 _ propanol
B.P. (^118 0) C^ B.P. (^108 0) C B.P.^830 C M.Wt = 74 M.Wt = M.Wt = 74 74
Soluble in water
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1.7 CHEMICAL REACTIONS OF ALCOHOLS
Alcohols acts both as nucleophiles as well as electrophiles. The bond between O- H is broken when alcohols react as nucleophiles and the bond between C-O is broken when they react as electrophiles. The chemical properties of any given aliphatic alcohol depend on the nature of the alkyl group and on the properties of the hydroxyl group. Based on the cleavage of O-H and C-OH bonds, the reactions of alcohols may be divided into two groups:
(A) Reactions involving cleavage of O-H bond
1. Acylation of alcohol: When alcohol reacts with acylhalide and anhydride substitution of hydrogen atom by acyl group is known as acylation of alcohols.
ROH + CH 3 COCl ROCOCH 3 +HCl
ROH + (CH 3 CO) 2 O ROCOCH3 + (^) CH 3 COOH
( B) Reaction involving fission of R—OH bond (cleavage of C—O bond): The reactions involving R – OH bond with cleavage of C – O bond are as follow
1. Dehydration: (a) Intramolecular dehydration (forming alkene): Alcohols undergo dehydration to form unsaturated hydrocarbon on treating with a protic acid e.g., con. H 2 SO 4 or H 3 PO 4 , or catalysts such as anhydrous ZnCl 2 or Al 2 O 3. In this reaction the OH and an H groups removes from an adjacent carbons. Since water is removed from the alcohol, this reaction is known as a dehydration reaction (or an elimination reaction ). Secondary and tertiary alcohols are dehydrated under much milder conditions. The conditions for dehydrating alcohols depend closely on the structure of individual alcohols.
For primary alcohols, the conditions required are conc. sulphuric acid and temperature of 170^0 C.
