Alcohol
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ALCOHOL Introduction It is an organic compound in which a hydroxyl group (–OH) is attached to an alkyl group. General molecular formula of alcohol is CnH2n+1OH and structurally it can be represented as
The hydroxyl group is functional group and all chemical properties of alcohol depend on it. Nomenclature and Structures Alcohols are named based on alkyl group to which hydroxyl group is attached. Some of the alcohols with their common name, IUPAC name and their structure are listed below
Alcohols are classified as 10, 20, or 30 depending upon the type of carbon to which hydroxyl group is attached. Alcohols are also named as monohydric, dihydric or trihydric depending upon the number hydroxyl groups attached to the alkyl group.
Physical properties Lower alcohols such as methanol ethanol etc are colorless toxic liquids with characteristic smell. The boiling points of the alcohols increase as the number of carbon atoms increases. The patterns in boiling point reflect the patterns of intermolecular forces and it is same as alkanes. The boiling point increases with the increase in molecular mass. The boiling point decreases with increase in branching. The boiling point of an alcohol is always much higher than that of the alkane with the same number of carbon atoms. This is due to hydrogen bonding. Alcohols are more soluble in water in comparison to alkane. This is again due to hydrogen bonding. Other wise solubility also shows the same trend as shown by alkane. Hydrogen bonding occurs between molecules where there is a hydrogen atom attached to one of the very electronegative elements - fluorine, oxygen or nitrogen. In case of alcohols, hydrogen bonds are set up between the slightly positive hydrogen atoms and lone pairs on oxygen in other molecules. Synthesis of Alcohols Alcohol can be prepared by various methods. Any one of the methods listed below can be used depending on the type and yield of alcohols required.
Reduction of carbonyl group: Carbonyl group is present in aldehydes, ketones and carboxylic acids. All these compounds can be reduced by either using direct hydrogen in presence of a catalyst or by using a reducing agent.
Both NaBH4 and LiAlH4 can be used to reduce aldehyde and ketones. Reduction of aldehydes will give primary alcohols while reduction of ketones will give secondary alcohols. In all the four reaction shown above hydride ion acts as a nucleophile.
Reducing agent attacks carbonyl carbon, breaks the double bond on oxygen, and forms alcohol. The hydride ion preference to carbonyl carbon is based on the electron donating tendency of the group attached to carbonyl carbon. More easily the electron is donated less likely the hydride ion gets attacked and forms alcohol. The tendency to form alcohol can be graded as
Reduction using Grignard Reagents(R – MgX) Grignard reagent is an organometallic compound having general formula as R – MgX (X = F,Cl, Br, I) These are highly polarized species due to polar bond between the metal and carbon. Carbon is more electronegative than the magnesium hence carbon becomes partially negative. The polarized carbon metal bond and a partially negative charge on the carbon make it a strong nucleophile and a base. Grignard reagent also reacts with C = N, S = O, N = O. It
deprotonates O – H, N – H, S – H. Grignmard reagent is made in ether. Carbon of carbonyl is positively charged as oxygen of carbonyl group is more electronegative than carbon. Thus it acts as an electrophile. The incoming nucleophile carbon of Grignard reagent attacks the carbonyl group carbon.
While adding to carbonyl group, organic part of Grignard reagent attaches itself to carbon and magnesium to oxygen of carbonyl group. The product is a weak salt of acidic alcohol which is easily converted to alcohol by giving it an acidic bath. Primary alcohol is produced by the reaction between formaldehyde and Grignard reagent.
Secondary alcohol is obtained by treating Grignard reagent with all other aldehydes other than formaldehyde.
Tertiary alcohol is produced due to the reaction between Grignard reagent and ketones.
In all the above reactions carbon chain is extended by one carbon. Grignard reagent therefore can be used to synthesize organic compounds in ascending order. i.e. methane to ethane or propane to butane etc. Several other methods for synthesis of alcohol such as hydration of alkene , oxymercuration-demercuration, hydroboration, and nucleophilic substitution have been discussed in the section of alkenes.
Acidic Nature of Alcohol The alcohols have polar structure in which oxygen has excess of electrons and the hydrogen of alcohols can be released. This is the reason why proton making alcohols are acidic in nature. However alcohols are weaker acids than water. Acidity of alcohols in descending order is methyl > i0 > 20 > 30
Methyl group is better electron donor when compared to hydrogen. As alkyl group increases, electron density on oxygen atom increases making it more and more negative. The increasing negative charge makes release of proton more difficult and acidic nature of alcohols reduces. Oxidation of Alcohol Oxidation is addition of oxygen, addition of halogens or loss of hydrogen while reduction is loss of oxygen, addition of hydrogen, or loss of halogen. Although there are other definitions as well but for the time being we will stick to these definitions. Oxidizing agents are the species with excess of oxygen while reducing agents have less oxygen atoms but high amount of hydrogen. Examples of oxidizing agents are K2Cr2O7, O2, Br2, H2CrO4 and reducing agent are H2, NaBH4, LiAlH4 Sequence of oxidation and reduction of various functional groups is given below:
Nucleophilic reactions of Alcohol The oxygen atom on alcohol has lone pair of electron which makes it basic as well as nucleophilic. In presence of strong acid, alcohol acts as a base and accepts proton.
Protonation of alcohols converts –OH into H2O which is a better leaving group. It also makes carbon atom more positive and more susceptible to nucleophilic attack and so SN2 as well as SN1 reaction become possible. The reaction is used to synthesize alkyl halide using SN2 mechanism by primary and secondary alcohol.
Halide ion shown in the above reaction comes from acid. The function of the reaction is to produce a substrate in which leaving group is a weakly basic water rather than a strongly basic hydroxide ion. Tertiary alcohols use SN1 mechanism. Reaction between tertbutyl alcohol and hydrochloric acid is taken as example.
In the first step, alcohol accepts a proton from the acid forming a protonated alcohol. It then dissociates in to water and forms a carbocation.
Carbocation in third step reacts with nucleophile and halide ion. The acid in this case helps in the formation of carbocation.
The reaction is carried out in presence of acid and high concentration of halide ion. The large quantity of halide ions helps stabilization of carbocation. The alcohols can be converted to alkyl halide by PCl5, PBr3 , PI3 by SN2 mechanism. The result with tertiary alcohols is poor. Alkyl halide can also be produced by thionyl chlorides to obtain additionally sulfur dioxides.
Preparation of Mesylates and Tosylates
A sulfonate ion is an ion that contains the -S(=O)2-O− functional group. The general formula is RSO2O−, where R is some organic group. They are conjugate bases of sulfonic acids. Sulfonates, being weak bases, are good leaving groups in Sn1, Sn2, E1 and E2 reactions.
Alcohols react with sulfonyl chlorides to form ester called sulfonate. These reaction involves cleavage of O – H bond of alcohols. The reaction is nucleophilic substitution of type SN2 where alcohol acts as nucleophile.
The tosylates and mesylates are very good leaving groups, and weak bases. The reactions in which tosylates and mesylates act as a leaving group follow SN1or SN2 mechanism. A nucleophile alcohol shows elimination reaction as well as addition reaction. Addition reaction has been discussed in alkene section and elimination will be discussed in subsequent section.
Pinacols Rearrangement These are dihydric alcohols which are completely substituted such as 2, 3 dimethyl 2, 3 butanediol. These result from the dehydration by mineral acid and subsequent rearrangement form ketones. The reaction is known as pinacol rearrangement. The reaction takes the following steps. In first step the hydroxyl group is protonated and removed by acid to form carbocation. In second step protonated hydroxyl group is removed as water and carbocation is formed.
Carbocation rearranges it self and gets attached to different carbons forming protonated ketone.
Ketone is formed by loss of proton from protonated ketone.
Ethers Ethers are compounds in which two groups are attached to oxygen atom. To name the ether we name the two groups attached to oxygen followed by the word ether.
Properties of Ether a) Di-methyl ether and ethyl-methyl ether are gases at room temperature. Rests all are liquids with pleasant order. b) Lower ethers are volatile and highly flammable. c) Boiling point follow the trend of alkane, alkene etc. d) Boiling point is lower than the corresponding alcohol due to lack of hydrogen bonding. e) Ethers are slightly soluble in water. Ether can form hydrogen bonding with water as well as with other compounds which contain a hydrogen atom attached to N, O, F atom. f) Ethers are very good organic solvents. g) Chemically ether reacts with concentrated HI and HBr to give alcohol and alkyl halide.
The concentration of acid should be proper, because excess of acid will react with alcohol and form alkyl halide. h) Ether can be oxidized to peroxides.
i) In general ethers highly nonreactive compounds. Epoxides (Oxiranes) Epoxides are cyclic ethers with ethereal oxygen as part of three membered ring.
Epoxides are more reactive than ethers due to strain created by small ring. An epoxide is protonated by acid. The protonated epoxide can then be attacked by any of the nucleophile reagents.
The protonated epoxide now can be used to synthesize a compound with two functional groups such as glycol by treating protonated epoxide with water. The reaction is anti-addition.
The protonated epoxide reaction with alcohol yields a compound which is ether along with alcohol.
The protonated epoxide reaction with HBr which is a compound having a bromine and alcohol as functional group.
Relative Acidity of Functional Group Acidic character of various functional groups in increasing order is as follows:
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The hydroxyl group is functional group and all chemical properties of alcohol depend on it. Nomenclature and Structures Alcohols are named based on alkyl group to which hydroxyl group is attached. Some of the alcohols with their common name, IUPAC name and their structure are listed below
Alcohols are classified as 10, 20, or 30 depending upon the type of carbon to which hydroxyl group is attached. Alcohols are also named as monohydric, dihydric or trihydric depending upon the number hydroxyl groups attached to the alkyl group.
Physical properties Lower alcohols such as methanol ethanol etc are colorless toxic liquids with characteristic smell. The boiling points of the alcohols increase as the number of carbon atoms increases. The patterns in boiling point reflect the patterns of intermolecular forces and it is same as alkanes. The boiling point increases with the increase in molecular mass. The boiling point decreases with increase in branching. The boiling point of an alcohol is always much higher than that of the alkane with the same number of carbon atoms. This is due to hydrogen bonding. Alcohols are more soluble in water in comparison to alkane. This is again due to hydrogen bonding. Other wise solubility also shows the same trend as shown by alkane. Hydrogen bonding occurs between molecules where there is a hydrogen atom attached to one of the very electronegative elements - fluorine, oxygen or nitrogen. In case of alcohols, hydrogen bonds are set up between the slightly positive hydrogen atoms and lone pairs on oxygen in other molecules. Synthesis of Alcohols Alcohol can be prepared by various methods. Any one of the methods listed below can be used depending on the type and yield of alcohols required.
Reduction of carbonyl group: Carbonyl group is present in aldehydes, ketones and carboxylic acids. All these compounds can be reduced by either using direct hydrogen in presence of a catalyst or by using a reducing agent.
Both NaBH4 and LiAlH4 can be used to reduce aldehyde and ketones. Reduction of aldehydes will give primary alcohols while reduction of ketones will give secondary alcohols. In all the four reaction shown above hydride ion acts as a nucleophile.
Reducing agent attacks carbonyl carbon, breaks the double bond on oxygen, and forms alcohol. The hydride ion preference to carbonyl carbon is based on the electron donating tendency of the group attached to carbonyl carbon. More easily the electron is donated less likely the hydride ion gets attacked and forms alcohol. The tendency to form alcohol can be graded as
Reduction using Grignard Reagents(R – MgX) Grignard reagent is an organometallic compound having general formula as R – MgX (X = F,Cl, Br, I) These are highly polarized species due to polar bond between the metal and carbon. Carbon is more electronegative than the magnesium hence carbon becomes partially negative. The polarized carbon metal bond and a partially negative charge on the carbon make it a strong nucleophile and a base. Grignard reagent also reacts with C = N, S = O, N = O. It
deprotonates O – H, N – H, S – H. Grignmard reagent is made in ether. Carbon of carbonyl is positively charged as oxygen of carbonyl group is more electronegative than carbon. Thus it acts as an electrophile. The incoming nucleophile carbon of Grignard reagent attacks the carbonyl group carbon.
While adding to carbonyl group, organic part of Grignard reagent attaches itself to carbon and magnesium to oxygen of carbonyl group. The product is a weak salt of acidic alcohol which is easily converted to alcohol by giving it an acidic bath. Primary alcohol is produced by the reaction between formaldehyde and Grignard reagent.
Secondary alcohol is obtained by treating Grignard reagent with all other aldehydes other than formaldehyde.
Tertiary alcohol is produced due to the reaction between Grignard reagent and ketones.
In all the above reactions carbon chain is extended by one carbon. Grignard reagent therefore can be used to synthesize organic compounds in ascending order. i.e. methane to ethane or propane to butane etc. Several other methods for synthesis of alcohol such as hydration of alkene , oxymercuration-demercuration, hydroboration, and nucleophilic substitution have been discussed in the section of alkenes.
Acidic Nature of Alcohol The alcohols have polar structure in which oxygen has excess of electrons and the hydrogen of alcohols can be released. This is the reason why proton making alcohols are acidic in nature. However alcohols are weaker acids than water. Acidity of alcohols in descending order is methyl > i0 > 20 > 30
Methyl group is better electron donor when compared to hydrogen. As alkyl group increases, electron density on oxygen atom increases making it more and more negative. The increasing negative charge makes release of proton more difficult and acidic nature of alcohols reduces. Oxidation of Alcohol Oxidation is addition of oxygen, addition of halogens or loss of hydrogen while reduction is loss of oxygen, addition of hydrogen, or loss of halogen. Although there are other definitions as well but for the time being we will stick to these definitions. Oxidizing agents are the species with excess of oxygen while reducing agents have less oxygen atoms but high amount of hydrogen. Examples of oxidizing agents are K2Cr2O7, O2, Br2, H2CrO4 and reducing agent are H2, NaBH4, LiAlH4 Sequence of oxidation and reduction of various functional groups is given below:
Nucleophilic reactions of Alcohol The oxygen atom on alcohol has lone pair of electron which makes it basic as well as nucleophilic. In presence of strong acid, alcohol acts as a base and accepts proton.
Protonation of alcohols converts –OH into H2O which is a better leaving group. It also makes carbon atom more positive and more susceptible to nucleophilic attack and so SN2 as well as SN1 reaction become possible. The reaction is used to synthesize alkyl halide using SN2 mechanism by primary and secondary alcohol.
Halide ion shown in the above reaction comes from acid. The function of the reaction is to produce a substrate in which leaving group is a weakly basic water rather than a strongly basic hydroxide ion. Tertiary alcohols use SN1 mechanism. Reaction between tertbutyl alcohol and hydrochloric acid is taken as example.
In the first step, alcohol accepts a proton from the acid forming a protonated alcohol. It then dissociates in to water and forms a carbocation.
Carbocation in third step reacts with nucleophile and halide ion. The acid in this case helps in the formation of carbocation.
The reaction is carried out in presence of acid and high concentration of halide ion. The large quantity of halide ions helps stabilization of carbocation. The alcohols can be converted to alkyl halide by PCl5, PBr3 , PI3 by SN2 mechanism. The result with tertiary alcohols is poor. Alkyl halide can also be produced by thionyl chlorides to obtain additionally sulfur dioxides.
Preparation of Mesylates and Tosylates
A sulfonate ion is an ion that contains the -S(=O)2-O− functional group. The general formula is RSO2O−, where R is some organic group. They are conjugate bases of sulfonic acids. Sulfonates, being weak bases, are good leaving groups in Sn1, Sn2, E1 and E2 reactions.
Alcohols react with sulfonyl chlorides to form ester called sulfonate. These reaction involves cleavage of O – H bond of alcohols. The reaction is nucleophilic substitution of type SN2 where alcohol acts as nucleophile.
The tosylates and mesylates are very good leaving groups, and weak bases. The reactions in which tosylates and mesylates act as a leaving group follow SN1or SN2 mechanism. A nucleophile alcohol shows elimination reaction as well as addition reaction. Addition reaction has been discussed in alkene section and elimination will be discussed in subsequent section.
Pinacols Rearrangement These are dihydric alcohols which are completely substituted such as 2, 3 dimethyl 2, 3 butanediol. These result from the dehydration by mineral acid and subsequent rearrangement form ketones. The reaction is known as pinacol rearrangement. The reaction takes the following steps. In first step the hydroxyl group is protonated and removed by acid to form carbocation. In second step protonated hydroxyl group is removed as water and carbocation is formed.
Carbocation rearranges it self and gets attached to different carbons forming protonated ketone.
Ketone is formed by loss of proton from protonated ketone.
Ethers Ethers are compounds in which two groups are attached to oxygen atom. To name the ether we name the two groups attached to oxygen followed by the word ether.
Properties of Ether a) Di-methyl ether and ethyl-methyl ether are gases at room temperature. Rests all are liquids with pleasant order. b) Lower ethers are volatile and highly flammable. c) Boiling point follow the trend of alkane, alkene etc. d) Boiling point is lower than the corresponding alcohol due to lack of hydrogen bonding. e) Ethers are slightly soluble in water. Ether can form hydrogen bonding with water as well as with other compounds which contain a hydrogen atom attached to N, O, F atom. f) Ethers are very good organic solvents. g) Chemically ether reacts with concentrated HI and HBr to give alcohol and alkyl halide.
The concentration of acid should be proper, because excess of acid will react with alcohol and form alkyl halide. h) Ether can be oxidized to peroxides.
i) In general ethers highly nonreactive compounds. Epoxides (Oxiranes) Epoxides are cyclic ethers with ethereal oxygen as part of three membered ring.
Epoxides are more reactive than ethers due to strain created by small ring. An epoxide is protonated by acid. The protonated epoxide can then be attacked by any of the nucleophile reagents.
The protonated epoxide now can be used to synthesize a compound with two functional groups such as glycol by treating protonated epoxide with water. The reaction is anti-addition.
The protonated epoxide reaction with alcohol yields a compound which is ether along with alcohol.
The protonated epoxide reaction with HBr which is a compound having a bromine and alcohol as functional group.
Relative Acidity of Functional Group Acidic character of various functional groups in increasing order is as follows:
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