ORGANIC CHEMISTRY - CHAPTER 9: FURTHER REACTIONS OF ALCOHOLS AND THE CHEMISTRY OF ETHERS

Important Concepts

1. The reactivity of ROH with alkali metals to give alkoxides and hydrogen follows the order R = CH3 > primary > secondary > tertiary.
2. In the presence of acid and a nucleophilic counterion, primary alcohols undergo SN2 reactions. Secondary and tertiary alcohols tend to form carbocations in the presence of acid, capable of E1 and SN1 product formation, before and after rearrangement.
3.  Carbocation rearrangements take place by hydride and alkyl group shifts. They usually result in interconversion of secondary carbocations or conversion of a secondary into a tertiary carbocation. Primary alkyl oxoniumions can rearrange by a concerted process consisting of loss of water and simultaneous hydride or alkyl shift to give secondary or tertiary carbocations.
4. Synthesis of primary and secondary haloalkanes can be achieved with less risk of rearrangement by methods using inorganic esters.
5. Ethers are prepared by either the Williamson ether synthesisor by reaction of alcohols with strong nonnucleophilic acids. The first method is best when SN2 reactivity is high. In the latter case, elimination (dehydration) is a competing process at higher temperatures.
6.  Crown ethersand cryptandsare examples of ionophores,polyethers that coordinate around metal ions, thus rendering them soluble in hydrophobic media.
7. Whereas nucleophilic ring opening of oxacyclopropanesby anions is at the less substituted ring carbon according to the rules of the SN2 reaction, acid-catalyzed opening favors the more substituted carbon, because of charge control of nucleophilic attack.
8. Sulfur has more diffuse orbitals than does oxygen. In thiols,the S – H bond is less polarized than the O – H bond in alcohols, thus leading to diminished hydrogen bonding.Because the S – H bond is also weaker than the O – H bond, the acidityof thiols is greaterthan that of alcohols.
9.  Note on color use: Throughout the main parts of the text, beginning in Chapter 6, reacting species in mechanisms and most examples of new transformations are color coded red for nucleophiles, blue for electrophiles, and green for leaving groups. Color coding is notused in exercises, summaries of new reactions, or chapter-end problems

From "Organic Chemistry" Textbook of VOLLHARDT & SCHORE