CHEM 10052 - Introduction to Organic Chemistry
Summer 1999 - Exam #3 Review
Classes of Compounds
| Class | General Formula | Polarity | H-Bonding | Major Reactions |
| Alkyl Halide | R-X, (X = F, Cl, Br, I) | Polar | No | Substitution |
| Alcohol | R-O-H | Polar | Yes | Substitution, Elimination, Oxidation |
| Phenol | (C6H5)-OH | Polar | Yes | Oxidation |
| Thiol | R-S-H | Weakly polar | No | Formation of disulfide |
| Ether | R-O-R' | Weakly polar | No | – |
Substitution reactions of Alkyl Halides
Halide atoms (typically abbreviated as X, where X = F, Cl, Br, I) are very electronegative, and the C-X bond is polar (Cd+–Xd-). The carbon atom is thus susceptible to attack by nucleophiles(Nuc), giving rise to substitution products.
R-X + Nuc- ¾® R-Nuc + X-
The mechanism depends primarily on the amount of room around the carbon center. Methyl and primary alkyl halides react almost exclusively via an SN2 mechanism, while steric hinderance forces tertiary alkyl halides to undergo SN1 reactions. Secondary alkyl halide can undergo either mechanism depending on the strength of the attacking nucleophile and the solvent. (We will ignore solvent effects). Strong nucleophiles tend to react via an SN2 mechanism, while weaker nucleophiles react via an SN1 mechanism. (More information about these mechanisms, including visualization of these reactions, is available). As a crude characterization, we will typically classify anions (negatively charged ions) as strong nucleophiles and neutral molecules as weak nucleophiles. More accurate differentiation of nucleophile strength can be predicted using the following trends.
| Property | Trend | Example |
| Row (of periodic table) | Going across row, nucleophilicity decreases |
C- > N- > O- > F- |
| Column (of periodic table) | Going down column, nucleophilicity increases |
O- < S- < Se- < Te- |
| Charge | Negative ions more nucleophilic than neutral molecules |
CH3OH < CH3O- |
Classification of Alcohols
Classification based on |
 |
 |
 |
 |
| methanol | 1° (primary) | 2° (secondary) | 3° (tertiary) |
Elimination and Oxidation Reactions of Alcohols
In both elimination and oxidation reactions of alcohols, two hydrogen atoms are lost. The first is always the OH hydrogen, but the second comes from different C-H locations depending on the reaction. For elimination reactions, the H is lost from a carbon atom next to the carbon containing the -OH group. For oxidation, the H is lost from the "same" carbon.
| Elimination |  |
|
| Oxidation |  |
The mechanism for elimination reactions of alcohols is either E1 (for tertiary alcohols) or E2 (for methanol and primary alcohols). This first step for both mechanisms is protonation (addition of H+) to the alcohol -OH group. In the E1 mechanism, H2O is eliminated from this oxonium ion to give a carbocation, followed by attack of the C-H hydrogen which is lost to give the alkene. In the E2 mechanism, loss of H2O and the C-H hydrogen occur simultaneously.
If more than one elimination product is possible, Zaitsev's rule states that the more substituted product is expected to be the major organic product. For example:
CH3–CH2–CH(OH)–CH3 ¾® CH3–CH=CH–CH3 (major) + CH3–CH2–CH=CH2 (minor)
Reactions
| Class | Type | General Reaction | Comments |
| Alkyl halide | Substitution | R-X + Nuc- ¾® R-Nuc + X- | SN1 or SN2 |
| Alcohol | Substitution | R-OH + HX ¾® RX
+ H2O R-OH + SOCl2 ¾® RCl R-OH + PBr3 ¾® RBr |
SN1 or SN2 |
| Elimination | R2CH–C(OH)R2 + H2SO4 ¾® R2C=CR2 + H2O | E1 or E2 | |
| Oxidation | RCH2OH + [O] ¾®
RC(=O)H RC(=O)H + [O] ¾® RC(=O)OH R2CHOH + [O] ¾® R2C(=O) R3COH + [O] ¾® NR |
1° ® aldehyde 1° ® ® acid 2° ® ketone 3° ® no Rxn. |
|
| Quinone | Redox | hydroquinone + [O] ¾® quinone | (reversible) |
| Thiol | Oxidation | 2 R3C–S–H + [O] ¾® R3–S–S–R3 | disulfide |
Acid Base Reactions
| Class | Reaction | Comments |
| Alcohol | R-O-H + base ¾® R-O- (alkoxide) + HBase+ | very strong base required |
| Alcohol | R-O-H + H+ ¾® R-OH2+ (oxonium salt) | first step in E1 & E2 mechanism |
| Thiol | R-S-H + base ¾® R-S- + HBase+ | base can be OH- |