Organic Chemistry DAT Notes
Organic Chemistry DAT Studying Notes
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SN1 vs. SN2
- SN1: 1) reactivity: 3o halides > 2o > 1o
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 > 1o > 2o > 3o
- Reactivity of leaving groups: I > Br> Cl > F
- Subsitution usually favored over elimination
E1 vs. E2
- It is a two-step process of elimination: ionization and deprotonation.
- 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 SN1 and E1 pathways are competing, the E1 pathway can be favored by increasing the heat.
- 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 SN2 and also between E1 and SN1. 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
- Tollens (+) = aldehyde or alpha-hydroxy ketone
- 2,4 DNP (+) = carbonyls
- Wittig – makes C=O C=R
- HIO4 – cleaves diols carbonyls
- 3o 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 (3o)
- Allenes (-C=C=C-) can be chiral if there are 2 diff. substituents on each side.
- Sulfonation = reversible rxn
- E-donating group more activating group = 1st atom is more e-negative
- (ex. –OCH3 > -CH2CH3)