Chapter 7

A structural element that decreases the number of hydrogens in the molecule by two • Multiple bonds and rings are elements of unsaturation

To calculate: Find number of hydrogens if they were saturated (2n +2; n is # of carbons), subtract the actual number of hydrogens, then divide by 2

Halogens replace hydrogen atoms in hydrocarbons, so when calculating unsaturations, count halides as hydrogen atoms

Oxygen does not change the C:H ratio, so ignore oxygen in the formula

Nitrogen is trivalent, so it acts like half a carbon. Add the number of nitrogen atoms when calculating unsaturations

• Find the longest continuous carbon chain that includes the double-bonded carbons • -ane changes to -ene • Number the chain so that the double bond has the lowest possible number. • In a ring, the double bond is assumed to be between Carbon 1 and Carbon 2

• If high priority groups are on the same side, the name is Z (for zusammen)

• If high priority groups are on opposite sides, the name is E (for entgegen)

(most stable) tetrasubstituted > trisubstituted > disubstituted > monosubstituted

cis < geminal < trans isomer (most stable)

• A bridged bicyclic compound cannot have a double bond at a bridgehead position unless one of the rings contains at least eight carbon atoms

• Strong base abstracts H+ as double bond forms and X- leaves from the adjacent carbon • Tertiary and hindered secondary alkyl halides give good yields

more substituted alkene

less substituted alkene

Bulky bases produce more of the Hofmann product If the substrate is prone to substitution, a bulky base can minimize the amount of substitution. Large alkyl groups on a bulky base hinder its approach to attack a carbon atom (substitution), yet it can easily abstract a proton (elimination).

Where both substitution and elimination are possible, using a bulky base gives a clean elimination result.

Similar to E2 reaction, except Br2 is removed from adjacent carbons instead of H–Br Bromines must also be anti-coplanar (E2) Use NaI in acetone, or Zn in acetic acid

Use NaI in acetone, or Zn in acetic acid

High temperature and weak/bulky bases will favor elimination over substitution.

E1

• Use concentrated sulfuric or phosphoric acid to protonate the alcohol, and make it a good leaving group • Rest of the reaction is the same as an E1 reaction • To increase the yield of the reaction, you can remove the low-boiling alkene as it forms to shift the equilibrium (the reaction is reversible) Mildly acidic solvent: favors elimination

Critical Rules for Mechanisms: • Do not form basic species (neg. charge) in acidic media • Do not form acidic species in basic media

–Substitution products are formed (SN1-nucleophile adds to carbocation) –Amount of substitution depends on temperature and nature of solvent –Low temperature: substitution favored –High Temperature: elimination favored

Greater s character leads to an increase in electronegativity