Chapter amines content is of physical properties
This will surely help you: https://nyaspubs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1749-6632.1965.tb45397.x
To answer this question first I will tell you about the hydrophobic effect which means is that nonpolar molecules tend to self-aggregate in the presence of aqueous solution. The enthalpic contribution to hydrophobic interactions is due to the water molecules occupying lipophilic binding sites, hence consequently unable to form hydrogen bonds with the receptor.
Now,you will clearly understood that why alkyl part is unable to form h- bond.