Question

1). Why do some oxygen containing hydrocarbons form hydrogen bonds and others do not?

2). A functional group R would attatch itself to which carbon of an n-propyl group?

Answer 1

The formation of H- bonds with O containing hydrocarbons depends much on the linking of oxygen atom with a particular C atom in the hydrocarbon chain. The following considerations should be kept in mind while making out the behaviour / solubility of a hydrocarbon molecule with water molecules .

1) O is a small sized electronegative atom favoring formation of H- bonds

2) Alkyl groups are electon releasing group

3) When all the carbon atoms in a hydrocarbon molecule along with an oxygen atom are bonded with sigma bonds the electon density over oxygen atom is still more increased , due to +ive inductive effect ( as a result of electron releasing tendency of alkyl group) .This causes Oxygen atom of the hydrocarbon molecule to become more negatively polar. The negative polarity of the oxygen atom attracts positively polar H atom in a water molecule resulting into the formation of H bonds between the two molecule . Example R-O-H

R----->>>-----O ------H

But in case of ethers the oxygen atom is bonded to two alkyl groups on both the sides / ends , hence ,the alkyl groups ( on both sides of oxygen atom of an ether molecule ) gets positive polarity while the Oatom becomes negatively polar. This results into a low polarity of the ether molecule ( obviously due to attractive forces between the two alkyl groups and the oxygen atom in the molecule itself ) . This low polarity helps lower ethers in their partial solubility in water due to some Hydrogen bondings. R₂ O ( delta negative )-------H (delta positive )-O-H However, increase in molecular weight shows the usual decrease in solubility of ethers in water.

Now, let us have a look over a oxygen containing hydrocarbon where oxygen atom is doubly bonded through one sigma and one pi bond, such as aldehydes or ketones.( ie . carbonyl compounds ) Here also the molecule gets positive and negative polarities due to electron relesing tendencies of the alkyl group / groups as shown below-

= C (delta positive ) =O ( delta negative ) It results into carbonyl compounds having dipole- dipole attrractions stonger than van der Waal's but weaker than hydrogen bonding. Ketones have more dipolar-dipolar interaction than aldehydes. This is due to presence of two electron repelling groups ( alkyl ) on carbon atom of the carbonyl group. However lower members of aldehydes and ketones up to four C atoms are soluble in water due to Hydrogen bonding. Because they can form hydrogen bonding with water molecules. However the solubility decreases with increase in molecular mass.. viz.

  

H₂ C (delta positive ) = O (delta negative )---------H (delta positive ) - O (delta negative) - H(delta positiive )------

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( 2 ) The attachment or bonding of the functional group R would also depend on the structure of the molecule of which the propyl group forms a part. Similar considerations relating to +I or -I effect and the nature / type of bonds present in the structure of the substrate molecule come into play. Presence of electronegative atoms such as O, Cl, Br, I, N greatly influence the bonding of alkyl group with a particular C atom of n- propyl group.