SN1 and SN2 Rxns..I need help

[sully677]

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I am learning to identify whether SN1 or SN2 is better. Is there any helpful websites or tips you have as to how to identify these rxns?

 

[sully677]

bump.

 

[drizzt3117]

I am learning to identify whether SN1 or SN2 is better. Is there any helpful websites or tips you have as to how to identify these rxns?

www.backsideattack.com

 

[BrokenGlass]

I am learning to identify whether SN1 or SN2 is better. Is there any helpful websites or tips you have as to how to identify these rxns?

A nucleophile is a compound that is either negatively charged or has a region of high electron density (full negative charge, partial negative charge, lone pairs, or p bonds). A nucleophile shares an electron pair to form a new covalent bond. The following structural features influence the driving force of the business end of the nucleophile to donate electron density to an electrophile.

• Resonance (most influential)
• Atomic radius (same column; the effect is reversed in protic solvents)
• Electronegativity (same period)
• Inductive effect (electron withdrawing or electron donating)
• Formal charge

A leaving group is the portion of the molecule that departs along with the pair of electrons that was the bond between the leaving group and some other atom (usually carbon). The more effectively the leaving group can stabilize this new electron pair, the more eagerly it will leave. An electrophile bears a leaving group. The nucleophile and the leaving group could be part of the same molecule.

An electrophile is a compound that is either positively charged or has a region of low electron density (full positive charge, partial positive charge, or an open octet).

Protic solvent is a hydrogen bond donor. In common organic solvents, the hydrogen bond donor is an O-H bond. Proticity is easy to tell from structure: simply look for O-H, O-N and O-F bonds. Hydrogen bonding is only significant when the nucleophile bears a negative charge or its lone pairs reside on a second-row element (most commonly nitrogen or oxygen). Therefore, protic solvents don't always decrease nucleophilicity.

Aprotic solvent does not have a hydrogen atom that can be shared by hydrogen bonding.

When choosing between substitution and elimination mechanisms consider the mechanisms in the order below. We consider the non-carbocation mechanisms (E2 and SN2) first because they generally have lower activation energy (Ea) for the rate-determining step (rds) than the carbocation mechanisms (SN1, E1). Furthermore, weconsider E2 before SN2 (except in the case of primary alkyl halides) because base/nucleophile moves past b hydrogens before approaching carbon bearing the leaving group.

E2 Requirements

• strong base (e.g. -OR, -NR2)
• moderate or better leaving group
• periplanar (hydrogen atom b to the leaving group)

Basicity is not strongly influenced by solvent polarity for E2 reactions.


SN2 Requirements

• good nucleophile (loss of resonance inhibits nucleophilicity)
• moderate or better leaving group
• carbon undergoing substitution not 3° (less hindered electrophiles are better)
• polar solvent (polar aprotic preferred)

E1/SN1 Requirements

• Moderate or better leaving group
• Stable carbocation (carbocation more stable than 1°, i.e. not CH3, not 1&#176😉
  • Alkyl groups are weak electron donors (inductive effect)
  • Resonance stabilization is more important than one degree of substitution. Less electronegative atoms are more willing to share electrons
  • 3° with resonance is best
• Polar solvent
  • Stabilizes leaving group and carbocation
  • Stabilizes transition state more than reactants thereby decreasing Ea
• E1 needs a b-hydrogen

No reaction if none of the requirement sets above is met.


In E2 reactions, when the base (e.g. -OC(CH3)3) or leaving group (NR3 or SR2) is large, the least substituted alkene is favored (Hofmann orientation). Otherwise, the most substituted alkene is favored (Zaitsev orientations). A sterically hindered base such as K+-OC(CH3)3 also helps favor elimination over substitution. In an E2 reaction, the C-H and C-LG bonds being broken must be periplanar (dihedral angle of 0 or 180 degrees). In a cyclohexane ring, this can only be achieved when both bonds are axial (anti-periplanar)

SN2 reaction rates in a protic solvent (e.g. methanol) are lower because of hydrogen bonding. H-bonding causes loss of nucleophilicity and thereby reduces the reaction rate, but not always by the same amount. For example, I-, Br-, Cl-, and F- all become worse nucleophiles in protic solvents, but I- takes the smallest hit. In other words, atomic radius effect gets reversed in protic solvents: the bigger the atomic radius the better the nucleophile in protic solvents. Overall, the best solvent choice for most SN2 reactions is polar aprotic. Polar protic is usually acceptable, but the reaction is slower due to hydrogen bonding. Nonpolar solvents are usually not useful for SN2 reactions because they don't dissolve most anionic nucleophiles. Many, but not all SN2 reactions are concerted. For example, an SN2 reaction can be two steps if the nucleophile must be deprotonated before attacking the electrophile.




How to make an SN2 reaction faster

• Decrease steric hindrance at the carbon bearing the leaving group
• Improve the nucleophile (e.g. it can suffer less H-bonding)
• Improve the leaving group
• Make the solvent aprotic, if it's not already
• Change to a less polar solvent

How to make an SN2 reaction slower

• Increase steric hindrance at the carbon bearing the leaving group
• Make the solvent more protic
• Make the leaving group worse
• Make the nucleophile worse (remember that in protic solvents, atomic radius effect of business end of nucleophile gets reversed)

E1 and SN1 are considered together because they share the same rds: the ionization of the carbon-leaving group bond (rds). Ionization leading to a more stable carbocation occurs more readily. When a leaving group leaves, a bond is lost and no new bonds are formed, so carbocation formation is energetically expensive. In E1 reactions, the most stable alkene is favored. An SN1 reaction can formthree products: a pair of stereoisomers when the carbocation captures a nucleophile and the third product when the carbocation rearranges and then captures a nucleophile. In many SN1 reactions, the nucleophile is also the solvent (solvolysis).


How to make an SN1 reaction slower

• Use a less polar solvent
• Use a poorer leaving group
• Use less substitution

How to make an SN1 reaction faster

• Use a more stable carbocation (e.g. 3° with resonance)
• Use a better leaving group (e.g. I-)
• Use a more polar solvent (SN1 reactions are fastest in polar protic solvents)

We need a polar solvent to run substitution reactions. SN1 desperately needs the polar solvent to stabilize the carbocation and also to assist ionization of the leaving group to form a carbocation. SN2 needs a polar solvent to dissolve the nucleophile.



Two significant differences between an SN1 and SN2

· An SN2 reaction proceeds with inversion of stereochemistry, whereas an SN1 reaction leads to a mixture of stereoisomers.
· An SN2 reaction follows bimolecular kinetics, whereas an SN1 reaction follows unimolecular kinetics

Determining the reaction type from products

If the products are given and they contain no new p bonds, then it's not an elimination reaction. If products were obtained through resonance, then it's a carbocation mechanism. If the stereochemistry is not inverted, it's not an SN2 reaction (unless inverted an even number of times). If rearrangement occurred, it's not E2 or SN2.

If the molecular formula of the product(s) is given, compute double bond equivalents (DBE), also known as degree of unsaturation.

Note: CH3I always undergoes SN2. It cannot undergo E2 since there are no b -hydrogens. It cannot undergo E1/SN1because carbocation is not stable enough.

DISCLAIMER: I wrote this a while back and it may contain errors. Use at your own risk.

 

[link2swim]

Here is some quick ways to determine if SN1 or SN2:

SN2 reactions will generally only occur if there is two or less carbons attached to the main carbon, and also they generally have strong nucleophiles(strong=negative charge) (nucleophile= thing replacing and getting bonded to the carbon in this reaction)


SN1 reaction generally occur with two or more carbons attached to the main carbon, and also they generally have weak nucleophiles (neutral= weak)


Some other main differences are that SN1 generally occur in polar protic and SN2 generally occur in polar aprotic. Also SN2 reactions occur in ONE step, and SN1 reactions occur in TWO steps.


If you want to know more about them read your O-chem book.

 

[TuxedoBrawl]

d


http://www.backsideattack.com/

 

[Revenant]

I can't access that site 🙁 . Are you sure it's working?

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[JaggerPlate]

SN2 is always more favorable (when compared to SN1 ... not necessarily for elimination reactions though) ... simply because the carbo cation step is very unfavorable(which makes it the rate determining step in SN1), high activation energy, etc etc. Remember though that SN2 requires a low degree of substitution such as leaving groups attached to methyl groups, or primary carbons ... where as SN1 requires a high degree of substitution (secondary with resonance or tertiary) because it pulls electron density from what it is connected to. Also, SN1 doesn't care about bad vs good nucleophile bc it's so reactive .... Those are just a few simple fact I always use to keep them separated in my mind.