SN2, SN1

SN: Substitution Nucleophilic
1/2: involving 1/2 reactants

SN2 & SN1

Most crucial:

• nucleophile attacks substrate from the other side where leaving group leaves;
• SN2: as a result, easiness: methyl > primary > secondary > tertiary from steric hinderance perspective;
• SN1: bottleneck: leaving group leaves, or, carbocation formation: easy to tell tertiary carbon would make it easier since other groups are carbon: if it's hydrogen then focal carbon would not be so positive, thus makes it harder for leaving group to leave. In other words, tertiary > secondary > primary for leaving group to leave, so is for SN1 reaction to occur.

Nucleophile & Electrophiles & Solvents

• Nucleophile
  • SN2: the nucleophile displaces the leaving group, meaning it must be strong enough to do so. Often, this means that the nucleophile is charged – if not, then it must be a strong neutral nucleophile. That being said, pay attention to sterics as well, as a very bulky nucleophile will be unable to do an sn2 reaction.
  • SN1: nucleophile tends to be uncharged and weaker, as it is attacking a carbocation.
• Electrophiles
  • SN2: depends on steric hinderance; Primary > secondary > tertiary
  • SN1: depends on easiness of carbocation formation; Tertiary > secondary > primary
• Solvents:
  • Sn1 reactions tend to happen in polar, protic solvents, because they can stabilize the carbocation charge better through their strong solvating power.
  • SN2: tend to happen in polar, aprotic solvents. This is because they are polar enough to dissolve the nucleophile and allow the reaction to proceed, but do not have the ability to hydrogen bond or as strong solvating power as the solvents for sn1 reactions.

	SN2	SN1
Electrophiles	depends on steric hinderance; Primary > secondary > tertiary	depends on easiness of carbocation formation; Tertiary > secondary > primary
Nucleophiles	strong, OH-	weak, e.g. CH3OH, H2O
Solvents	DMSO(dimethylsulfoxide)	water, alcohol
definition	leaving group leaves the same time when nucleophile gives electron to carbon	leaving group leaves slowly, then in 2nd step nucleophile reacts
rate	R = k[electrophile][nucleophile]	R = k[electrophile]
chirality inversion	Yes	No

Visual process & chirality inversion

sn2-sn1.png

Nucleophilicity

Nucleophilic: have extra electron to give away
Nucleophilicity: ability for atom/ion/molecule to give "extra" electron
e.g. fluoride, chloride, hydro ion. etc.

Both SN2 and SN1 would like to happen in polar solvent

	Protic solvent	Aprotic solvent
character	directs bond H	no direct bonds H
e.g.	water(H-O-H), alcohol(R-O-H)	diethyl ether(CH2CH3OCH2CH3)
analysis	nucleophile like F-, would easily hog + from solvent; small electron orbit too: hard to react for SN2	the opposite; no free H+ in aprotic solvent: easy for nucleophile to react
rank	F- < Cl- < Br- < I-(fluoride < chloride < bromide < iodide)	F- > Cl- > Br- > I-

Protic: can give proton(H⁺)

References:

1. ChemTalk SN1 SN2
2. DMSO solvents
3. diff SN2 SN1