FreeMCATPrep Logo

More Than 475 Free MCAT Questions with Detailed Answers!

Click HERE for your Random Question from our MCAT Question A Day Archive

Organic Chemistry - Alcohols

All alcohols undergo hydrogen bonding. Because they undergo hydrogen bonding, alcohols tend to have higher boiling points than alkanes or alkenes of similar molecular weight. Also because of their hydrogen bonding small sized alcohols with carbon chains of 3 carbons or fewer are soluble in water. Alcohols can lose a hydrogen ion from the OH-group attached to their carbon chain. That’s another way of saying that alcohols can act as acids. You should know that some alcohols are more acidic than others. In order to tell which alcohol is more acidic, look at the alcohol and see whether it’s a primary, secondary or tertiary alcohol. Then remember this when it comes to alcohol, the order of acidity is primary more than secondary and secondary more than tertiary. 

Alcohols can lose a whole molecule of water. An alcohol can lose its OH-group plus another atom of hydrogen and that means it’s losing a molecule of H2O. When an alcohol or any other molecule loses some molecule of H2O, we say it’s been dehydrated. When alcohols undergo dehydration, they formed alkenes. Some alcohols are dehydrated more readily than others. The order in which alcohols are willing to give up water and become an alkene is just the opposite of the order of their willingness to give up hydrogen. So, when talking about how willing an alcohol is to be dehydrated remember this order; tertiary more than secondary and secondary more than primary. 

Alcohols can lose an H2 molecule and when they do, they’re said to be oxidized. When an alcohol molecule loses H2 and is therefore oxidized, it ends up with the double bonded oxygen. Just where that double-bonded oxygen will be depends on whether the alcohol was a primary, secondary, or tertiary alcohol. When oxidized primary alcohols become aldehydes and secondary alcohols become ketones. Remember that the common oxidizing agents used are KMNO4 and K2CR2O7. You should know that it’s hard to oxidize a tertiary alcohol. When a tertiary alcohol is subjected to oxidation, a carbon and hydrogen are lost. The resulting ketone has a carbon chain that is shorter by one carbon than the original alcohol. An alcohol can have its OH replaced by a halide like chlorine, bromine or iodine. If the alcohol is a primary alcohol, the halide goes right on to the carbon that had the OH-group attached to it. This is called a substitution reaction. This is a one step process in which the halide knocks off the leading group in the rate-determining step. 

A different reaction occurs with tertiary alcohols. This is a two-step process which leads to the formation of a carbocation in the rate-determining step. Sometimes, rearrangement occurs when a secondary alcohol is halogenated to form a more stable carbocation. Both reactions are known as substitution reactions because chlorine substitutes for an OH group. So, when do we use SN1 and SN2? SN1 favors tertiary alcohols while SN2 is most likely to occur with primary alcohols. Secondary alcohols use either mechanisms but generally favor SN1. The only other things you need to know about substitution reactions is that for SN1, the rate depends only on the concentration of the alcohol while the rate of SN2 depends on concentration of both the alcohol and the halide. Know also that an inversion occurs during SN2 and rearrangement often occurs during SN1.