Why alchohols have greater boiling point then isomeric ethers meritnation
Answers
Alcohols are compounds in which one or more hydrogen atoms in an alkane have been replaced by an -OH group.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:
Note: If you aren't confident about naming organic compounds, then you really ought to follow this link before you go on.
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The different kinds of alcohols
Alcohols fall into different classes depending on how the -OH group is positioned on the chain of carbon atoms. There are some chemical differences between the various types.
Primary alcohols
In a primary (1°) alcohol, the carbon which carries the -OH group is only attached to one alkyl group.
Note: An alkyl group is a group such as methyl, CH3, or ethyl, CH3CH2. These are groups containing chains of carbon atoms which may be branched. Alkyl groups are given the general symbol R.
Some examples of primary alcohols include:
Notice that it doesn't matter how complicated the attached alkyl group is. In each case there is only one linkage to an alkyl group from the CH2 group holding the -OH group.
There is an exception to this. Methanol, CH3OH, is counted as a primary alcohol even though there are no alkyl groups attached to the carbon with the -OH group on it.
Secondary alcohols
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:
Tertiary alcohols
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:
Physical properties of alcohols
Boiling Points
The chart shows the boiling points of some simple primary alcohols with up to 4 carbon atoms.
They are:
They are compared with the equivalent alkane (methane to butane) with the same number of carbon atoms.
Notice that:
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 patterns in boiling point reflect the patterns in intermolecular attractions.
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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.
The hydrogen atoms are slightly positive because the bonding electrons are pulled away from them towards the very electronegative oxygen atoms.
Note: If you want to be fussy, the diagram is slightly misleading in that it suggests that all of the lone pairs on the oxygen atoms are forming hydrogen bonds. In an alcohol that can't happen. Taking the alcohol as a whole, there are only half as many slightly positive hydrogen atoms as there are lone pairs. At any one time, half of the lone pairs in the total liquid alcohol won't have hydrogen bonds from them because there aren't enough slightly positive hydrogens to go around.
In the diagram, to show the 3-dimensional arrangement, the wedge-shaped lines show bonds coming out of the screen or paper towards you. The dotted bonds (other than the hydrogen bonds) show bonds going back into the screen or paper away from you.
In alkanes, the only intermolecular forces are van der Waals dispersion forces. Hydrogen bonds are much stronger than these and therefore it takes more energy to separate alcohol molecules than it does to separate alkane molecules.
That's the main reason that the boiling points are higher.
The effect of van der Waals forces . . .
. . . on the boiling points of the