Organic Chem ALCOHOLS

ALCOHOLS Alcohols are compounds in which one or more hydrogen atoms in an alkane have been replaced by an -OH group. For the purposes of UK A level, we will only look at compounds containing one -OH group. For example:

P r i m a r y a lc lc o h o l s  

In a primary primar y (1°) alcohol, the carbon which carries the t he -OH group is only attached to one alkyl group Some examples of primary alcohols include:

S e c o n d a r y a lc lc o h o l s  

In a secondary (2°) alcohol, the carbon with the -OH group attached is joined directly to t w o  alkyl groups, which may be the same or different. Examples:

Tertiary alcoh ols 

In a tertiary (3°) alcohol, the carbon atom holding the -OH group is attached directly to   alkyl groups, which may be any combination of same or different. t h r e e  alkyl

Examples:

Hydrogen bonding 

Hydrogen bonding occurs between molecules where you have a hydrogen atom attached to one of the very electronegative elements - fluorine, oxygen or nitrogen. In the case of alcohols, there are hydrogen bonds set up between the slightly positive hydrogen atoms and lone pairs on oxygens in other molecules.

on the comparison between alkanes and alcohols:

Even if there wasn't any hydrogen bonding or dipole-dipole interactions, the boiling point of the alcohol would be higher than the corresponding alkane with the same number of carbon atoms. Compare ethane and ethanol:

Ethanol is a longer molecule, and the oxygen brings with it an extra 8 electrons. Both of these will increase the size of the van der Waals dispersion forces and so the boiling point. If you were doing a really fair comparison to show the effect of the hydrogen bonding on boiling point it would be better to compare ethanol with propane rather than ethane. The length would then be much the same, and the number of electrons is exactly the same

The small alcohols are completely soluble in water. Whatever proportions you mix them in, you will get a single solution. However, solubility falls as the length of the hydrocarbon chain in the alcohol increases. Once you get to four carbons and beyond, the fall in solubility is noticeable, and you may well end up with two layers in your test tube.

MANUFACTURE OF ALCOHOLS alcohols from alkenes The manufacture of ethanol from ethene Ethanol is manufactured by reacting ethene with steam. The catalyst used is solid silicon dioxide coated with phosphoric(V) acid. The reaction is reversible.

Only 5% of the ethene is converted into ethanol at each pass through the reactor. By removing the ethanol from the equilibrium mixture and recycling the ethene, it is possible to achieve an overall 95% conversion. to make an alcohol from propene, CH 3CH=CH2. there are two different alcohols which might be formed:

The major product is based on the Markovnikoff’s rule for asymmetrical addition.(The addition.(The rich gets richer)

Making ethanol by fermentation fermentation This method only  applies  applies to ethanol. You can't make any other alcohol this way.

The process The starting material for the process varies widely, but will normally be some form of starchy plant material such as maize (US: corn), wheat, barley or potatoes. A comparison of fermentation with the direct hydration of ethene Fermentation

Hydration of ethene

Type of process

A batch process. Everything is put into a container and then left until fermentation is complete. That batch is then cleared out and a new reaction set up. This is inefficient.

A continuous flow process. A stream of reactants is passed continuously over a catalyst. This is a more efficient way of doing things.

Rate of reaction

Very slow.

Very rapid.

Quality of product

Produces very impure ethanol which needs further processing

Produces much purer ethanol.

Reaction conditions

Uses gentle temperatures and atmospheric pressure.

Uses high temperatures and pressures, needing lots of energy input.

Use of resources

Uses renewable resources based on plant material.

Uses finite resources based on crude oil.

Dehydration of alcohols using aluminium oxide as catalyst The dehydration of ethanol to give ethene This is a simple way of making gaseous alkenes like ethene. If ethanol vapour is passed over heated aluminium oxide powder, the ethanol is essentially cracked to give ethene and water vapour.

Dehydration of alcohols using an acid catalyst The acid catalysts normally used are either concentrated sulphuric acid or concentrated phosphoric(V) acid, H 3PO4. Concentrated sulphuric acid produces messy results. Not only is it an acid, but it is also a strong oxidising agent. It oxidises some of the alcohol to carbon dioxide and at the same time is reduced itself to sulphur dioxide. Both of these gases have to be removed from the alkene. The dehydration of ethanol to give ethene Ethanol is heated with an excess of concentrated sulphuric acid at a temperature of 170°C. 170°C. The gases produced are ar e passed through sodium hydroxide solution to t o remove the carbon dioxide and sulphur dioxide produced from side reactions

The reaction between sodium and ethanol Compare the two:

Sodium ethoxide is just like sodium hydroxide, except that the hydrogen has been replaced by an ethyl group. Sodium hydroxide contains OH - ions; sodium ethoxide contains CH3CH2O- ions.

Reactions involving hydrogen halides The general reaction looks like this:

Reaction with hydrogen chloride Tertiary alcohols react reasonably rapidly with concentrated hydrochloric acid, but for primary or secondary alcohols the reaction rates are too slow for the reaction to be of much importance. This is an SN 1 reaction

A tertiary alcohol reacts if it is shaken with with concentrated hydrochloric acid at room temperature. A tertiary halogenoalkane (haloalkane or alkyl halide) is formed

Replacing -OH by bromine Rather than using hydrobromic acid, you usually treat the alcohol with a mixture of sodium or potassium bromide and concentrated sulphuric acid. This produces hydrogen bromide which reacts with the alcohol. The mixture is warmed to distil off the bromoalkane.

Replacing -OH by iodine In this case the alcohol is reacted with a mixture of sodium or potassium iodide and concentrated phosphoric(V) acid, H 3PO4, and the iodoalkane is distilled off. The mixture of the iodide and phosphoric(V) acid produces hydrogen iodide which reacts with the alcohol.

Phosphoric(V) acid is used instead of concentrated sulphuric acid because sulphuric acid oxidises iodide ions to iodine and produces hardly any hydrogen iodide

Reacting alcohols with phosphorus halides Reaction with phosphorus(III) chloride, PCl 3 Alcohols react with liquid phosphorus(III) chloride (also called phosphorus trichloride) to make chloroalkanes.

Reaction with phosphorus(V) chloride, PCl 5 Solid phosphorus(V) chloride (phosphorus pentachloride) reacts violently with alcohols at room temperature, producing clouds of hydrogen chloride gas it is used as a t e s t f o r - O H g r o u p s  in    in organic chemistry.

To show that a substance was an alcohol, you would first have to eliminate all the other things which also react with phosphorus(V) chloride. For example, carboxylic acids (containing the -COOH group) react with it (because of the -OH in -COOH), and so does water (H-OH).If you have a neutral liquid not contaminated with water, and get clouds of hydrogen chloride when you add phosphorus(V) chloride, then you have an alcohol group present.

Other reactions involving phosphorus halides Instead of using phosphorus(III) bromide or iodide, the alcohol is usually heated under reflux with a mixture of red phosphorus and either bromine or iodine. The phosphorus first reacts with the bromine or iodine to give the phosphorus(III) halide.

These then react with the alcohol to give the corresponding halogenoalkane which can be distilled off.

Reacting alcohols with sulphur dichloride oxide (thionyl chloride) The sulphur dichloride oxide reacts with alcohols at room temperature to produce a chloroalkane. Sulphur dioxide and hydrogen chloride are given off. Care would have to be taken because both of these are poisonous.

OXIDATION OF ALCOHOLS Oxidising the different types of alcohols The oxidising agent used in these reactions is normally a solution of sodium or potassium dichromate(VI) acidified with dilute sulphuric acid. If oxidation occurs, the orange solution containing the dichromate(VI) ions is reduced to a green solution containing chromium(III) ions. The electron-half-equation for this reaction is

Primary alcohols Primary alcohols can be oxidised to either aldehydes or carboxylic acids depending on the reaction conditions. In the case of the formation of carboxylic acids, the alcohol is first oxidised to an aldehyde which is then oxidised further to the acid. P a r t ia ia l o x i d a t i o n t o a l d e h y d e s  

You get an aldehyde if you use an excess of the alcohol, and distil off the aldehyde as soon as it forms. The excess of the alcohol means that there isn't enough oxidising agent present to carry out the second stage. Removing the aldehyde as soon as it is formed means that it doesn't hang around waiting to be oxidised anyway! If you used ethanol as a typical primary alcohol, you would produce the aldehyde ethanal, CH3CHO. The full equation for this reaction is fairly complicated, and you need to understand about electron-half-equations in order to work it out.

In organic chemistry, simplified versions are often used which concentrate on what is happening to the organic substances. To do that, oxygen from an oxidising agent is represented as [O]. That would produce the much simpler equation:

It also helps in remembering what happens. You can draw simple structures to show the relationship between the primary alcohol and the aldehyde formed.

Full oxidation to carboxylic acids 

You need to use an excess of the oxidising agent and make sure that the aldehyde formed as the half-way product stays in the mixture.

The alcohol is heated under reflux with an excess of the oxidising agent. When the reaction is complete, the carboxylic acid is distilled off. The full equation for the oxidation of ethanol to ethanoic acid is: The more usual simplified version looks like this: Alternatively, you could write separate equations for the two stages of the reaction - the formation of ethanal and then its subsequent oxidation.

This is what is happening in the second stage:

Secondary alcohols Secondary alcohols are oxidised to ketones - and that's it. For example, if you heat the secondary alcohol propan-2-ol with sodium or potassium dichromate(VI) solution acidified with dilute sulphuric acid, you get propanone formed Using the simple version of the equation and showing the relationship between the structures:

Tertiary alcohols Tertiary alcohols aren't oxidised by acidified sodium or potassium dichromate(VI) solution. There is no reaction whatsoever. You need to be able to remove those two particular hydrogen atoms in order to set up the carbon-oxygen double bond.

To differentiate between primary n secondary alcohols Schiff's reagent is a fuchsin dye decolourised by passing sulphur dioxide through it. In the presence of even small amounts of an aldehyde, it turns bright magenta It must, however, be used absolutely cold, because ketones react with it very slowly to give the same colour. If you heat it, obviously the change is faster - and potentially confusing. While you are warming the reaction mixture in the hot water bath, you can pass any vapours produced through some Schiff's reagent.

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If the Schiff's reagent quickly becomes magenta, then you are producing an aldehyde from a primary alcohol. If there is no colour change in the Schiff's reagent, or only a trace of pink colour within a minute or so, then you aren't producing an aldehyde, and so haven't got a primary alcohol. Because of the colour change to the acidified potassium dichromate(VI) solution, you must therefore have a secondary alcohol.

You should check the result as soon as the potassium dichromate(VI) solution turns green - if you leave it too long, the Schiff's reagent might start to change colour in the secondary alcohol case as well.

You need to produce enough of the aldehyde (from oxidation of a primary alcohol) or ketone (from a secondary alcohol) to be able to test them. There are various things which aldehydes do which ketones don't. These include the reactions with Tollens' reagent, Fehling's solution and Benedict's solution

The presence of that hydrogen atom makes aldehydes very easy to oxidise. Or, put another way, they are strong reducing agents Because ketones don't have that particular hydrogen atom, they are resistant to oxidation. Only very strong oxidising agents like potassium manganate(VII) solution (potassium permanganate solution) oxidise ketones - and they do it in a destructive way, breaking carbon-carbon bonds

Under acidic conditions it is:

. . . and under alkaline conditions:

Using acidified potassium dichromate(VI) solution

A small amount of potassium dichromate(VI) solution is acidified with dilute sulphuric acid and a few drops of the aldehyde or ketone are added. If nothing happens in the cold, the mixture is warmed gently for a couple of minutes - for example, in a beaker of hot water. ketone

No change in the orange solution.

aldehyde

Orange solution turns green.

The orange dichromate(VI) ions have been reduced to green chromium(III) ions by the aldehyde. In turn the aldehyde is oxidised to the corresponding carboxylic acid. Using Tollens' reagent (the silver mirror test) Tollens' reagent contains the diamminesilver(I) ion, [Ag(NH 3)2]+. This is made from silver(I) nitrate solution. You add a drop of sodium hydroxide solution to give a precipitate of silver(I) oxide, and then add just enough dilute ammonia solution to redissolve the precipitate. To carry out the test, you add a few drops of the aldehyde or ketone to the freshly prepared reagent, and warm gently in a hot water bath for a few minutes. ketone

No change in the colourless solution.

aldehyde

The colourless solution produces a grey precipitate of silver, or a silver mirror on the test tube.

Aldehydes reduce the diamminesilver(I) ion to metallic silver. Because the solution is alkaline, the aldehyde itself is oxidised to a salt of the corresponding carboxylic acid. The electron-half-equation for the reduction of of the diamminesilver(I) ions to silver is:

Using Fehling's solution or Benedict's solution Fehling's solution and Benedict's solution are variants of essentially the same thing. Both contain complexed copper(II) ions in an alkaline solution.

Fehling's solution contains copper(II) ions complexed with tartrate ions in sodium hydroxide solution. Complexing the copper(II) ions with tartrate ions prevents precipitation of copper(II) hydroxide. Benedict's solution contains copper(II) ions complexed with citrate ions in sodium carbonate solution. Again, complexing the copper(II) ions prevents the formation of a precipitate - this time of copper(II) carbonate. Both solutions are used in the same way. A few drops of the aldehyde or ketone are added to the reagent, and the mixture is warmed gently in a hot water bath for a few minutes. ketone

No change in the blue solution.

aldehyde

The blue solution produces a dark red precipitate of copper(I) oxide.

Aldehydes reduce the complexed copper(II) ion to copper(I) oxide. Because the solution is alkaline, the aldehyde itself is oxidised to a salt of the corresponding carboxylic acid.