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- Chemistry S5 SB File Uploaded 28/01/22, 14:06
UNIT 5: ALCOHOLS AND ETHERS
UNIT 5: ALCOHOLS AND ETHERSKey unit competency:To be able to compare the physical and chemical properties of alcohols and ethers to their preparation methods, reactivity and uses.Learning objectives:• Distinguish between alcohols from other organic compounds by representing the functional group of alcohols• Classify primary, secondary and tertiary alcohols by carrying out the method of identification• Write the name of alcohols by using IUPAC system• Describe the physical properties of alcohols to other series of organic compounds• Carry out the method of preparation of alcohols• Describe the local process of making alcohol by fermentation.• Explain the effect of oxidation on urwagwa when it overstays• Compare the physical, chemical and the method of preparation of alcohols to ethers• State the use of ethers
5.1. Definition and nomenclature
5.1.1. DefinitionAlcohols are organic compounds that are derivatives of hydrocarbons where one or more hydrogen atoms of hydrocarbon is or are replaced by hydroxyl (-OH) group. They are represented by the general formula: CnH2n+1OH or ROH where R is a radical: alkyl group made by a chain of carbon atomsAlcohols are called monohydric if only one hydroxyl group is present (eg: CH3CH2-OH). Dihydric alcohols are those with two hydroxyl group (diol: vicinal and gem), trihydric (triols) and polyhydric are those with many – C-OH groups. The functional group attached is –OH group to any atom of carbon.5.1.2. NomenclatureAccording to IUPAC system, alcohols are named by replacing the final ‘‘e’’ of the parent hydrocarbon with ‘‘ol’’, then specify the position of -OH group before ending by ol
5.2. Classification and isomerism
Alcohols are classified as:Primary alcohols: These have only one alkyl group attached to the carbon carrying the –OH.
• Functional isomers:Except methanol which has one carbon, other alcohols are isomers with ethersanother chemical function of general formula R-O-R’ where R and R’are alkyl groups or aryl groups but not hydrogen.
5.3. Physical properties
a.Boiling pointsThe chart shows the boiling points of some simple primary alcohols and alkanes with up to 4 carbon atoms.
• The boiling point of an alcohol is always much higher than that of the alkane with the same number of carbon atoms.• The boiling points of the alcohols increase as the number of carbon atoms increases.• The boiling point of alcohols with branches is lower than that of unbranched alcohols with the same number of carbon atoms. This is because increased branching gives molecules a nearly spherical shape and the surface area of contact between molecules in the liquid. This results in weakened intermolecular forces and therefore in lower boiling points.• Tertiary alcohols exhibit the lowest boiling point than secondary and primary alcohols:• Primary alcohol > Secondary alcohol > Tertiary alcoholHighest boiling point lowest boiling pointThe patterns in boiling point reflect the patterns in intermolecular attractions: In the case of alcohols, there are hydrogen bonds set up between the slightly positive hydrogen atoms and lone pairs on oxygen in other molecules.
b.Solubility of alcohols in waterThe lower members of alcohols are completely soluble in water because mixed hydrogen bonds between water and alcohol molecules are formed. As the length of hydrocarbon group of the alcohol increases, the solubility decreases.c. VolatilityAlcohols are volatile and the volatility decreases as the molecular mass increases. Compared to alkyl halides, alcohols are less volatile. Polyalcohol are viscous or solids. Example: propane-1, 2, 3-triol (glycerine). This is due to stronger intermolecular forces than those of monoalcohols.
5.4. Alcohol preparations
b. From alkenes
Alkenes react with water in the presence concentrated sulphuric acid to yields
alcohols
c. From carbonyl compounds
When aldehydes and ketones are reduced by hydrogen in the presence of a
suitable catalyst like Pt, Ni or Pd, they form primary and secondary alcohols
respectively.
d. From esters
Esters on hydrolysis in the presence of mineral acid or alkalis produce alcohols and
carboxylic acids.
e. From Grignard reagents
The reaction between carbonyl compound and Grignard reagent (alkyl magnesium
halides) produces an alcohol with more carbon atoms. The reaction is a nucleophilic addition on a carbonyl compound.
f. From primary amine to give primary alcohol
Primary amines react with nitrous acid to produce primary alcohols.
5.5. Preparation of ethanol by fermentation
This method is mainly used to prepare ethanol industrially. Ethanol is prepared from starch(e.g. maize, cassava, millet, sorghum) and sugar(e.g. banana juice, molasses) by fermentation process.Fermentation can be defined as any of many anaerobic biochemical reactions in which enzymes produced by microorganisms catalyse the conversion of on substance into another.
The ethanol obtained by fermentation process is only about 11%. This is made concentrated by distillation which converts it to about 95% ethanol. This on further distillation yields a constant boiling mixture whose composition does not change (an azeotropic mixture). Therefore, 100% ethanol is obtained by either:i. Adding quick lime which removes waterii. Distilling with of benzene as a third componentNote: Methanol can be prepared industrially by the reaction of carbon monoxideand hydrogen at 300 °C and a pressure of 200 atmospheres.
5.6.1. Oxidation
Aldehydes formed by oxidation of primary alcohols tend to undergo further oxidation to carboxylic acid.Ketones formed by oxidation of secondary alcohols are not further oxidised, unless if the oxidising agent is hot and concentrated in which case bonds around the –CO_ group are broken and two smaller carboxylic acids are formed.
Tertiary alcohols resist oxidation because they have no hydrogen atom attached on the functional carbon atom. Oxidation also occurs when the alcohol is in gaseous phase by used of silver or copper catalyst under 500 °C and 300 °C respectively; and the vapour of the alcohol is passed with air (oxygen) over heated silver.
An acidified potassium dichromate solution is turned from orange to green when it reacts with primary and secondary alcohols.Secondary alcohols having the following structure R-CHOH-CH3 only undergo oxidation, on treatment with iodine solution in the presence of sodium hydroxide to give yellow precipitate of tri-iodomethane.Note: This is a reaction which is characteristic of methyl ketones, CH3-CO-R’; but iodine here acting as an oxidizing agent first oxidizes the CH3-CHOH-R’toCH3-CO-R’ ; then the methyl ketone formed then gives the yellow precipitate of CHI3 (Iodoform). From the reaction involved we have the Iodoform test.
5.6.2. Reaction with sulphuric acid
5.6.4. Reaction with strong electropositive metals and metal hydroxides
5.6.5 Action of hydrohalic acids (HX)
Notice:i. Reaction with concentrated hydrochloric acid is catalyzed by anhydrous zinc chloride.ii. This reaction is called LUCAS test and is used to distinguish between simple primary, secondary or tertiary alcohols. In this reaction, the alcohol is shaken with a solution of zinc chloride in concentrated hydrochloric acid.Observations: Immediate cloudiness indicates presence of a tertiary alcohol. If the solution becomes cloudy within 5 minutes then the alcohol is a secondary one. Primary alcohol would show no cloudiness at room temperature since the reaction is very slow.For example all alcohols which are isomers of C4H10O can be distinguished by the LUCAS test.Alcohols are also transformed into halogenoalkanes using phosphorus halides and thionyl chloride.
5.7. Uses of alcohols
Ethanol is the alcohol found in alcoholic drinks. Alcoholic fermentation converts starch sugar into ethanol. For example grapes are used to produce wine, ripe banana to produce urwagwa, honey for spirits are obtained by distilling the ethanol –water product obtained when sugar is fermented.Drinking alcohol, i.e. the ethylic alcohol also called ethanol, is a normal social activity; but excess of it is dangerous for our health. Hence excess of alcoholic consumption must be avoided.For non-adult youth, consumption of alcohol in any form is illegal in Rwanda and many other countries.There are some alcoholic drinks produced in Rwanda and in the Region that are prohibited to be sold in Rwanda. However, alcohols have many other applications in daily life as indicated in the Table 5.1.
Ethanol produced by sugar cane fermentation has been used as alternative fuel to gasoline (petrol). It has been mixed with gasoline to produce gasohol.
1. Ethers are sparingly soluble in water but are soluble in organic solvents.2. The polar nature of the C-O bond (due to the electronegativity difference of the atoms) results in intermolecular dipole-dipole interactions.3. An ether cannot form hydrogen bonds with other ether molecules since there is no H to be donated (no -OH group).4. Their melting and boiling points increase with the increase in molecular mass because of increasing the magnitude of Van der Waal’s forces with size.5. The boiling points of ethers are much lower than those of alcohols of similar molecular mass. This is because of the intermolecular hydrogen bonding which are present in alcohols but are not possible in ethers.
Since they are saturated compounds and non-polar, they are relatively chemically inert reason why their chemical reactions are very few.
a. Ethers can act as the Lewis base due to the two non-bonded electron pair on oxygen to form coordinative bonds with Grignard reagent. This explains clearly why organ magnesium compounds are manipulated in ether solvent but not in water since in water, there is a reaction which generate alkanes.
Lower ethers are used as anesthesia since they produce inert local cooling when sprayed on a skin, ether are also used as local anesthesia for minor surgery operation.Lower ethers are volatile liquid which on evaporation produce low temperature they are therefore used as refrigerants.Ether itself is one of the most important organic solvents for fats, oils, resins, and alkaloids.
