Organic Chemistry: Structure Controls Reactivity
Organic Chemistry: structure controls reactivity
The OAT Organic Chemistry section tests bonding and hybridization, nomenclature, stereochemistry, substitution and elimination, addition, aromaticity, carbonyl chemistry, and spectroscopy by IR and NMR. The exam rewards predicting products and identifying functional groups from data.
Core rules
- Hybridization: carbon with four sigma bonds is sp3 and tetrahedral; three sigma bonds is sp2 and trigonal planar; two is sp and linear. Aromatic rings are planar, cyclic, conjugated, and follow Huckel's 4n+2 pi-electron rule.
- Stereochemistry: a chiral center has four different groups. Assign R/S by priority using atomic number, with the lowest priority pointing away. Enantiomers invert all centers; diastereomers invert some.
- Substitution and elimination: SN2 is one-step with backside attack, favored by methyl or primary substrates and polar aprotic solvents. SN1 proceeds through a carbocation, favored by tertiary substrates and polar protic solvents. E2 requires an anti-periplanar beta hydrogen and a strong base.
- Carbonyls: nucleophiles add to aldehydes and ketones; carboxylic acid derivatives undergo nucleophilic acyl substitution. Aldehydes are more reactive than ketones because of less steric hindrance and less alkyl donation.
- Spectroscopy: IR shows broad O-H near 3200-3600 cm-1 and C=O near 1700 cm-1. In 1H NMR, n+1 splitting occurs; chemical shift increases with deshielding.
Worked example
A compound with formula C3H7Br reacts with strong base to give propene as the major product. Which mechanism and orientation apply? A strong base favors E2. The beta hydrogen must be anti-periplanar to Br. Removing a beta hydrogen from C1 or C3 can give propene; only propene is possible here, so the major pathway is E2 rather than SN2.
Traps
- Calling a molecule chiral when it has a plane of symmetry.
- Using n+1 when protons are equivalent.
- Assuming SN1 for primary halides.
- Forgetting that aromaticity requires a continuous p-orbital ring.