Alcohols are compounds in which one or more hydrogen atoms in an alkane have been replaced by an -OH group.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:
In a primary (1°) alcohol, the carbon which carries the -OH group is only attached to one alkyl group Some examples of primary alcohols include:
In a secondary (2°) alcohol, the carbon with the -OH group attached is joined directly to two alkyl groups, which may be the same or different. Examples:
In a tertiary (3°) alcohol, the carbon atom holding the -OH group is attached directly to three alkyl groups, which may be any combination of same or different. Examples:
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, CH3CH=CH2. there are two different alcohols which might be formed:
The major product is based on the Markovnikoff’s rule for asymmetrical addition.(The rich gets richer) Making ethanol by fermentation
This method only 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
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
Quality of product
Produces very impure ethanol which needs further processing
Produces much purer ethanol.
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...
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