Nucleophilic Substitution

nucleophilic aliphatic substitution

The substitution of one group for another at a saturated, sp3-hybridized carbon atom, where the nucleophile attacks and replaces the leaving group.

Sn1

Two step mechanism where the formation of a carbocation intermediate occurs before the nucleophile attacks the substrate. First a heterolytic cleavage, then the nucleophile bonds to the carbocation.

The RDS is the first step (Rate = k1[R-L]) (first order rate constant)

Best for tertiary

Sn2

Concerted mechanism where the formation of a carbocation intermediate occurs at the same time the nucleophile attacks the substrate.

The single concerted step is the RDS (Rate = k1[R-L][Nu:]) (second order rate constant)

Best for primary, methyl

Sn1 vs. Sn2

Sn1 - tertiary

Sn2 - primary, methyl

Steric hinderance stops Sn2/makes it difficult

Infrared Spectroscopy

energy required to make bonds to move (stretch bend etc)

diff bonds have different frequency/absorb diff wavelengths to get them to stretch/bend and tells what the functional group is.

Halide Test + Analysis

Sodium iodide test- Sodium iodide + halide reacts to form iodide + sodium halide. Formation of precipitant means there is indeed a halogen (chloride). Sn2 mechanism.

Silver Nitrate test- Form silver halide + nitrate. Formation of precipitant means there is indeed a halogen (chloride). Sn1 mechanism.

The faster rate of precipitant from the tests means if the reaction is Sn2 or Sn1

Nucleophilic Substitution requirements (why HCl)

Leaving Group + Nucleophile

Why HCl- very strong acid thus a weak conjugate base, so it attacks the carbocation not pulling off a hydrogen. Turns our poor leaving group OH into a good leaving group (H20). Also it provides the nucleophile in Cl-. x3 more HCl than alcohol to push reaction forward since it is in equilibrium

wash vs extraction

A liquid-liquid wash is pulling what you dont want into the extraction solvent.

A liquid liquid extraction is pulling whatever compound you’re interested into the extraction solvent.

Need to know where the product that you want is at.

IR analysis

intensity- size of peak. symmetry of compound decreases intensity. a lot of aromatics = small peak. a lot of alkanes = very big peak

frequency- where the peak is and which bond is present

Functional region + Fingerprint region

Fingerprint region

1250 - 400 cm-1

if region peaks looks the same as the molecule being compared to it is probably that molecule.

Functional Group region

functional group region:

4000 - 1250 cm-1 functional group region

2850 - 3000 cm-1 is carbon hydrogen stretch

1650-1750 cm-1 is carbonyl

3200 - 3600 cm-1 is alcohol. intensity based on hydrogen bonding. more carbons less hydrogen bonding smaller peak/less intensity

General reaction and mechanism

2-methyl-2-butonal to 2-Chloro-2-methylbutane

Sn1

2nd step is RDS