|Lecture Notes: 17 June||
Thiol Nomenclature: Thiols are named as alcohols, but -ol is replaced with thiol . The -SH group is called a mercapto group. Thus an -SH group on an aromatic ring would be a mercapto substituent. Examples: HOCH2CH2SH = 2-Mercaptoethanol
Common names use Mercaptan: CH3CH2SH = Ehtyl mercaptan (= Ethanethiol)
Sulfides are named as ethers but with sulfide replacing ether and alkylthio replacing alkoxy in the name. Examples
Physical properties: Alcohols and phenols differ significantly from hydrocarbons (alkanes, alkenes, alkynes, and aromatics) and alkyl halides in their melting points and their boiling points. Thus, for short chain alcohols and for phenols we find much higher boiling points than one would expect on the basis of their sizes.
The most dramatic differences occur with the smallest molecules: methane (MW = 16), chloromethane (MW = 50), and methanol (MW = 32) with boiling points of -161.4°C, -23.7°C, and 64.7°C, respectively. And if we compare the chloromethane with an alkane of similar MW, butane (MW = 58), the boiling point is -0.5°C - thus the alkyl chlorides and alkanes have bp's which are similar and related to size. Alcohols obviously require a different explanation.
We explain the high bp's of alcohols with the existence of hydrogen bonds (H-bonds), this is the same explanation we see for the incredibly high boiling point of water (water has about 4 H-bonds per molecule, vs. about 2 for alcohols). Hydrogen bonds involve a very strong polar attraction combined with very small atomic radii, this results in a partial covalent bond being formed. This bond is much weaker than the covalent bonds common in organic molecules (i.e. 100 kcal/mol for O-H bonds vs. about 5 kcal/mol for a H-bond), but is significantly stronger than the weak van der Waals forces holding alkanes together (about 0.1 - 0.2 kcal/mol).
Note that as MW's increase the differences in properties diminish - all of the molecules begin to behave more like alkanes as the alkane portion become predominant.
Acidity of Alcohols: Alcohols are weakly acidic, as we might expect due to their similarity to water. They can also function as weak Lewis bases, becoming protonated to give oxonium ions:
Some pKa values: CH3CH2OH = 16.0; H2O = 15.7; CH3OH = 15.5; HCl = -7.0.
Reaction with Alkali and Alkaline Earth Metals: Alcohols react with active metals such as Na, K, Ca etc. much like water does, but somewhat less exciting:
Dehydration: Earlier we saw the dehydration of alcohols as a method of synthesizing alkenes with the products usually predicted by Zaitsev's rule - the more highly substituted alkene will be the major product. Recall that this reaction is acid catalyzed, commonly being carried out in sulfuric acid (tertiary alcohols reacting under mild conditions, primary and secondary requiring higher temperatures). Later we found that the mechanism of this reaction is an E1 elimination reaction:
Alkyl Halides from Alcohols: As seen earlier this is the best method for preparing alkyl halides. Tertiary alcohols react via an SN1 mechanism. Primary and secondary alcohols may be readily converted to alkyl halides by reaction with SOCl2 or PBr3. At first site these later reactions seems surprising. If they are going via an SN1 mechanism, then the reactions should go slowly. On the other hand -OH is a poor leaving group in SN2 reactions. This reaction is in fact enhanced by the reaction of the -OH leaving group with either SOCl2 (giving SO(OH)2 in two reactions) or PBr3 (giving H3PO3 in three reactions) to give a good leaving group in each case.
Last modified 17 June 2004