Organic Chemistry DAT Notes

Organic Chemistry DAT Studying Notes

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SN1 vs. SN2

• SN1: 1) reactivity: 3^o halides > 2^o > 1^o

2) rate = k[halide]

3) stereochemical result = partial inversion and racemization

4) accelerated by polar protic solvents (ex. H₂O & CH₃OH) – stabilize the carbocation

5) slow step = formation of carbocation

• SN2: 1) stereochemical result = 100% inversion

2) rate = k[halide][Nu]

3) backside attack of Nu on X^– (sp³ C  sp²  sp³)

4) best in polar aprotic solvents (ex. DMSO, HMPT, acetone)

5) best w/ strong Nu (ex. CN^–)

6) reactivity: methyl > 1^o > 2^o > 3^o

• Reactivity of leaving groups: I > Br> Cl > F
• Subsitution usually favored over elimination

E1 vs. E2

• E1
  • It is a two-step process of elimination: ionization and deprotonation.
    • Ionization, Carbon-halogen breaks to give a carbocation intermediate.
    • Deprotonation of the carbocation.
  • Typical of tertiary and some secondary substituted alkyl halides.
  • Reaction mostly occurs in complete absence of base or presence of only a weak base.
  • No antiperiplanar requirement.
  • E1 eliminations happen with highly substituted alkyl halides due to 2 main reasons.
    • Highly substituted alkyl halides are bulky, limiting the room for the E2 one-step mechanism; therefore, the two-step E1 mechanism is favored.
    • Highly substituted carbocations are more stable than methyl or primary substituted. Such stability gives time for the two-step E1 mechanism to occur.
  • If S_N1 and E1 pathways are competing, the E1 pathway can be favored by increasing the heat.
• E2
  • It is a one-step process of elimination with a single transition state.
  • Typical of secondary or tertiary substituted alkyl halides. It is also observable with primary alkyl halides if a hindered base is used.
  • Because E2 mechanism results in formation of a Pi bond, the two leaving groups (often a hydrogen and a halogen) need to be coplanar. Involves an antiperiplanar transition state has staggered conformation with lower energy.
  • Reaction often present with strong base.
  • In order for the pi bond to be created, the hybridization of carbons need to be lowered from sp³ to sp².

Elimination > Substitution conditions

The reaction rate is influenced by halogen‘s reactivity. There is a certain level of competition between elimination reaction and nucleophilic substitution. More precisely, there are competitions between E2 and S_N2 and also between E1 and S_N1. Substitution generally predominates and elimination occurs only during precise circumstances. Generally, elimination is favored over substitution when

• steric hindrance increases
• basicity increases
• temperature increases
• the steric bulk of the base increases for example Potassium tert-butoxide
• the nucleophile is poor

Lab Tests

• Tollens (+) = aldehyde or alpha-hydroxy ketone
• 2,4 DNP (+) = carbonyls

Rxn Info

• Wittig – makes C=O  C=R
  • (CH₃)₃-P⁺R
• HIO₄ – cleaves diols  carbonyls

Physical Characteristics

• 3^o alcohols dehydrate the fastest
• Dipole molecule required for IR signal.
• More conjugated = more UV absorption @ longest wavelength
• Low T = kinetic control = 1,2 addition
• Smallest heat of hydrogenation = more stable alkene (3^o)
• Allenes (-C=C=C-) can be chiral if there are 2 diff. substituents on each side.

Misc.

• Sulfonation = reversible rxn
• E-donating group  more activating group = 1^st atom is more e-negative
  • (ex. –OCH₃ > -CH₂CH₃)

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