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Alcohols are organic compounds that are formed when hydrogen present in a hydrocarbon, aliphatic and aromatic, gets replaced by the -OH group. Alcohol contains one or more than one hydroxyl (-OH) group and is directly attached to a carbon chain. Alcohols are most commonly used in industries; however, they are also used commonly in daily life. Alcohols contain at least one hydroxyl functional group (-OH) attached to a saturated hydrocarbon. For example, in methyl alcohol (molecular formula CH3OH) one saturated carbon atom, three hydrogen atoms and one hydroxyl functional group are present.
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Key Terms: Alcohols, Grignard Reagent, Beverages, Carbon Chain, Hydrogen, Functional Group, Carbon, Atom, Organic compounds, Alkyl Halides, Saturated hydrocarbon
What are Alcohols?
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Alcohols are not just intoxicating beverages which are often how the term is used in common parlance. They are substances that are not available to obtain freely in nature and alcohols are one of them. Since alcohols are not freely available in nature they are often obtained and there are various methods for obtaining alcohol some examples of these methods are hydroboration-oxidation, reduction of aldehydes and ketones, hydrolysis of Alkyl Halides, using a Grignard reagent, and so on.
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How are Alcohols Named?
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While alcohols like most compounds have a chemical name and also a common name. This common name is how the compound is referred to commonly. A common name for alcohol usually has two terms, first, one being derived from the alkyl group of the compound and the latter term is simply adding the term ‘alcohol’. For instance, the common name of CH3CH2OH is ethyl alcohol. Apart from the common name and formula, compounds are also named according to the system based by IUPAC, according to which alcohol is named by adding ‘ol’ to the parent alkene name of the compound.
Methods of Preparation of Alcohol
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Alcohol is widely used in various industries and is prepared using various methods. These methods are called methods of alcohol preparation. The various methods of alcohol preparation include:
- Grignard Reagents: Method of alcohol production by Grignard Reagents can help in preparing primary, secondary and tertiary alcohol. Grignard Reagents (RMgX) is added to carbonyl compounds which form an adduct. When hydrolysis of this adduct is done, it yields alcohol. Additionally, the reaction with methanol produces primary alcohol, similarly, a reaction with other aldehydes produces secondary alcohol, and finally, a reaction with ketones produces tertiary alcohol.
- Reduction of Aldehydes and Ketones: Alcohol in this process is produced by reducing aldehydes and ketones corresponding alcohols by the process of catalytic hydrogenation, which refers to the addition of hydrogen in presence of catalysts some commonly used examples of these catalysts include platinum, nickel. Similarly, by using reducing agents like lithium aluminium hydride (LiA1H4) for treating ketones and aldehydes. The former will yield secondary alcohols, while the latter will yield primary alcohol.
- Hydroboration-Oxidation: In this method, Alkanes are treated with diborane which gives trialkyl boranes. Which is then oxidised using hydrogen peroxide in presence of aq (NaOH) resulting in alcohol formation.
- Acid Catalysed Hydration/ Hydration of Alkenes: In this method of preparation, alcohols are prepared by alkenes reacting with water in the presence of catalysts. An indirect way of hydration happens when sulphuric acid is added to alkane followed by hydrolysis.
- Reduction of Carboxylic Acids and Esters: Using lithium aluminium hydridecarboxylic acids are reduced into primary alcohols.
- Oxymercuration and Demercuration of Alkanes: In the process of Oxymercuration and Demercuration of Alkenes and Mercuric acetate react along with water (H2O) and tetrahydrofuran to form mercury compounds which are alkyl. This is then further treated by Sodium borohydride (NaBh4). This is also a commonly used method for alcohol preparation.
- By Reaction of Nitrous Acid and Primary Amines: A reaction between nitrous acid and primary amines is also a method of alcohol preparation. An outcome of the reaction other than alcohol preparation is that odourless and colourless gas along with nitrogen is produced. This preparation method does not have a single equation for the reaction. For example, CH3NH2 gives CH3-O-N=O or CH3OCH3
CH3NH2 + 2HNO2 → CH3-O-N=O + 2H2O + N2
Or
2CH3NH2 + 2HNO2 → CH3OCH3 + 2N2 + 3H2O
One of the reaction mechanisms is given below:
- Fermentation: Probably a method that comes to mind most commonly when alcohol preparation is discussed in common parlance is the process of fermentation. Fermentation essentially is a slow process of decomposition of complex organic compounds into much simpler compounds. This happens because of the presence of enzymes. This is followed by the formation of gases like CO2 and CH4. Another important aspect of the fermentation process is that it has an anaerobic environment, meaning the process takes place in the absence of oxygen.
- Hydrolysis of Halides: In this process, alkyl halides are transformed into alcohol by using either water (H2O) or hydroxide (-OH) for nucleophilic substitution. This method commonly produces primary and secondary alcohols. The reaction process is
R-X + KOH → R-OH + KX
- Hydroformylation of Alkenes: Hydroformylation is a commonly used industrial method for alcohol preparation. In the process of hydroformylation of Alkenes, a reaction takes place between olefins which have a lower molecular weight, carbon monoxide (CO), and hydrogen. This takes place in the presence of a catalyst. An aldehyde is formed which then forms alcohol by hydrogenation.
Things to Remember
- Alcohols are organic compounds that are formed when hydrogen present in a hydrocarbon, aliphatic and aromatic, gets replaced by the -OH group. They are formed when one or more (-OH) groups are attached to carbon atoms.
- Alcohols are widely used in industries; they are also to be obtained and are not available in their form freely.
- There are different methods for obtaining/ preparation of alcohol some of them are: Grignard reagent method, hydroboration-oxidation method, acid-catalysed hydration method.
- Alcohols, phenols, and ethers are all organic compounds, however, have different characteristics.
- The process of Grignard Reaction is used to make primary, secondary, and tertiary alcohol.
- Alcohols are differentiated based on (-OH) and carbon atoms attached.
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Sample Questions
Ques. Give the formula and IUPAC name of the alcohols based on their common name for the following: (5 marks)
(i) Ethyl Alcohol
(ii) Methyl Alcohol
(iii) N-Propyl Alcohol
(iv) Isobutyl Alcohol
(v) Cyclohexyl Alcohol
(vi) Neopentyl Alcohol
Ans. Tabulated below are the IUPAC and Formula names of given alcohols:
| Common Name | IUPAC Name | Formula |
|---|---|---|
| ethyl alcohol | ethanol | CH3CH2OH |
| methyl alcohol | methanol | CH3OH |
| n-propyl alcohol | 1- propanol | CH3CH2CH2OH |
| isobutyl alcohol | 2-methyl-1-propanol | (CH3)2CHOH |
| cyclohexyl alcohol | cyclohexanol | cyclo-C6H11OH |
| neopentyl alcohol | 2,2-dimethyl-1-propanol | (CH3)3CCH2OH |
Ques. What is the oldest way of preparing alcohol? (3 marks)
Ans. One of the oldest ways of preparing alcohol is by sugar fermentation which produces the alcohol ethyl alcohol. This alcohol is produced in absence of air. This process of fermentation produces carbon dioxide. When the ferment comes in contact with air it oxidises which changes ethanol (the fermented mixture) into ethanoic acid.
Ques. Explain the process of preparing Methanol. Also mentions its important uses. (3 marks)
Ans. Methanol is prepared by the process of catalytic reduction of carbon monoxide. This is an industrial method of producing methanol under which carbon monoxide reacts with hydrogen which leads to the formation of Methanol. Some of the important uses of methanol are as follows:
- Methanol is widely used as an industry insolvent, it is often used to make dyes, resins.
- Methanol is also used for producing acetic acid and formaldehyde.
- Water and methanol are also often mixed and used in high-performance engines.
- Methanol is also considered a substitute for gasoline.
- It is also used as an antifreeze.
Ques. Which are the most commercially used alcohols, give reasons for why they are widely used? (3 marks)
Ans. The most commercially used alcohols are methanol and ethanol. These two are most commonly used because of their important features such as volatility, flammability. These features help in making ethanol and methanol useful as a solvent, fuel. They also help in making dye, resins. Other than this, methanol is also considered a better alternative fuel that does not contribute to pollution.
Ques. Differentiate between Ethanol and Methanol. (4 marks)
Ans. While Ethanol and Methanol both are variants of alcohol, they are different which are as follows:
| Ethanol | Methanol |
|---|---|
| Ethanol is a colourless liquid; it is also a volatile substance. It is formed by the process of ethylene hydration or even by fermentation of sugar using yeast. | Methanol is also a colourless liquid and also it is volatile and highly flammable. It can be produced by using renewable resources like biogas. |
| Ethanol is less acidic than water. | Methanol is more acidic than water. |
| The chemical formula of Ethanol is C2H5OH. | The chemical formula of Methanol is CH3OH. |
| Ethanol is widely used for making alcoholic beverages like beer, whisky. | Methanol is widely used as an industrial insolvent. |
| Ethanol is often used in alcoholic beverages like beer, however, consuming in high quantities can be poisonous. | Pure methanol is considered highly toxic. |
Ques. Differentiate between Aldehydes and Ketones. (3 marks)
Ans. The difference between Aldehydes and Ketones are as follows:
| Aldehydes | Ketones |
|---|---|
| Aldehydes have carbon atoms that share a double bond with oxygen and a single bond with a hydrogen atom. | Ketones have carbon atoms which are double bonded with oxygen atoms. |
| The general formula of aldehyde is CnH2nO. | The general formula of Ketones is CnH2nO. |
| Aldehydes are often naturally found in cells. | Ketones are found in sugars. |
| Some examples of Aldehydes are Formaldehyde, Acetaldehyde, Benzaldehyde. | Some examples of Ketones are methyl propyl ketone, methyl n-propyl ketone. |
Ques. Give some important uses of alcohol in industrial work. (3 marks)
Ans. Important uses of alcohol in industrial work are as follows:
- Alcohol is also used as it is most widely known in the form of alcoholic beverages, alcohols like ethyl (also called ethanol alcohol) are used in alcohol.
- Alcohols like ethanol are often considered a substitute for gasoline. It also contributes less to pollution.
- Ethanol is used as a solvent for perfume and cosmetics making.
- Methanal is used in producing plastics, paints, textiles, and cosmetics.
- Alcohol is also used as antifreeze.
- Alcohol is also used as an antiseptic and anti-disinfectant.
Ques. Differentiate between hydroboration-oxidation and acid- catalysed hydration. (3 marks)
Ans. Hydroboration-oxidation and acid- catalysed hydration are two methods of alcohol preparation methods. While both preparation methods involve the usage of alkenes, they also have different characteristics. The difference between the two is as follows:
| Hydroboration-Oxidation | Acid-Catalysed Hydration |
|---|---|
| Hydroboration-oxidation requires a two-step process: the first step is hydroboration and the second is oxidation. | Acid catalysed hydration requires a three-step process which includes: First, carbocation formation and carbocation are formed by the attack of H3O+. Secondly, Nucleophile H2O then attacks carbocation. And finally, with deprotonation, alcohol formation takes place H+ (hydrogen ion) will be eliminated. |
| The process of Alcohol preparation through hydroboration-oxidation which starts with hydroboration begins in an anti-Markovnikov way. | Acid-catalysed hydration provides alcohol in a Markovnikov. |
Ques. Differentiate between reduction of aldehydes and ketones and reduction of carboxylic acid and esters. (3 marks)
Ans. Reduction of aldehydes and ketones and reduction of carboxylic acid and esters are two methods of alcohol preparation methods. The difference between the two is as follows:
| Reduction of Aldehydes and Ketones | Reduction of Carboxylic Acid and Esters |
|---|---|
| The reduction agent used in the reduction of Aldehydes and ketones is NABH4 (Sodium Borate). | The reduction agent used in the Reduction of Carboxylic acid and esters process of alcohol preparation is liAlH4(Lithium aluminium hydride). |
| Aldehydes or ketones are more reactive to Nu than carboxylic acid and esters. | Carboxylic acid or esters are less reactive to Nu in comparison to aldehydes and ketones. |
Ques. Why are phenols more acidic than alcohol? (2 marks)
Ans. Alcohols and Phenols are both organic compounds. However, both have some differences in characteristics. One of these characteristics is that Phenols are more acidic than alcohol. Phenols in a reaction with sodium hydroxide (aqueous) produce phenoxide ions. These ions are an indicator of the acidity of phenols, which is rather higher than alcohol acidity level. The presence of an electron-withdrawing group leads to an increase in the acidity of phenol.
Ques. Differentiated between Alcohols, Phenols and Ethers. (5 marks)
Ans. Alcohols, Phenols, and Ethers are all organic compounds; however, they are different. Their differences are mentioned as follows:
| Alcohols | Phenols | Ethers |
|---|---|---|
| Alcohols have carbon atoms that hold at least one hydroxyl function group. It is formed by bonding a hydroxyl group with a saturated carbon atom. | Phenols are formed when a carbon atom with an aromatic ring is attached to a hydroxyl (-OH) group. | Ethers are formed when two oxygen atoms are attached to alkyl or aryl groups. |
| Alcohol is differentiated in two ways, one based on (-OH) and the basis of carbon atoms First differentiation has three kinds, namely, Monohydric Alcohol, Dihydric Alcohol and Trihydric Alcohol | Unlike Alcohol, Phenols have one set of differentiation, that is, they are differentiated by the number of hydroxyl groups attached to them. They are Monohydric Phenols, Dihydric Phenols and Trihydric Phenols | Ethers are classified as, Symmetrical Ethers and Unsymmetrical Ethers. rWhen the alkyl or aryl groups are attached to sides of the atom are the same. Also called simple ether or symmetrical ether. When the alkyl or aryl groups attached to sides of the atom are not the same, it is also called mixed ether or unsymmetrical ether. |
| Alcohols tend to be colourless and are less acidic and more towards being neutral. | Phenols are mildly acidic and it mostly exists in white crystalline solid. | Ethers are basic, in appearance, it is a colourless liquid. |
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