CHEM 2124 – Quick Review

CHEM 2124 – Quick Review Common inorganic reagents: CaCl2,

Na2SO4, HCl, H2SO4, HNO3, HC2H3O2 = CH3COOH, NaOH, KOH

Acetate salts or groups often abbreviated OAc. NaOAc, EtOAc

Et stands for ethyl, Me stands for methyl – EtOAc, EtOH, MeOH

CHEM 2124 – Quick Review Functional groupsAlkenes, alkynes, aromatic rings,

alcohols, haloalkanes (alkyl halides), esters, ethers, thiols, aldehydes, ketones, carboxylic acids, amines, amides, cyano (nitrile)

Polar vs. nonpolar molecules Intermolecular forces – hydrogen bonding,

dipole-dipole, London dispersion forces

CHEM 2124 – Quick Review Relative strengths of acids and bases

Acids:HCl > HF > HCOOH > CH3COOH > HCN > NH4

+ > CH3OH > H2O > CH3CH2OH

Lower pKa = stronger acid

Bases:Ethoxide ion > hydroxide ion > methoxide ion >

ammonia > cyanide ion > acetate ion > formate ion > fluoride ion > chloride ion

Lower pKb = stronger base

Conjugate bases of weaker acids are stronger bases.

CHEM 2124 – Quick Review Nucleophiles - “nucleus loving” -

electron rich (lone pair, often negative charge)Strong bases can act as strong nucleophilesConjugate acids of strong bases are weak

nucleophiles Ex: EtOH, H2O, NH3

Some weak bases are excellent nucleophiles Ex: CN

Ions with highly polarizable shells are excellent nucleophiles

Iodide > bromide > chloride >> fluoride SH > OH

CHEM 2124 – Quick Review SN1 vs. SN2

SN2 reactionsPromoted by strong nucleophilesPromoted by aprotic solvents (DMF, THF,

acetone)2nd order kinetics (rate = k[nuc]

[substrate])Backside attack gives inversion of

configuration (RS, SR)Methyl substrate > 1 > 2 not with 3

CHEM 2124 – Quick Review SN1 reactions

Favored with weak nucleophilesPromoted by protic solvents (H2O, alcohol)

Promoted by AgNO3 if halide substrate1st order kinetics (rate = k[substrate])Carbocation intermediateGives racemization (R 50/50 R + S)3 > 2 Always accompanied by E1 to some

extent

CHEM 2124 – Quick Review E1 and E2 E2

Favored with strong bases - especially bulky bases (Ex: tBu-O)

Solvent not a big factor3 > 2 > 12nd order kineticsCoplanar transition state required -

preferably anti-coplanarNo rearrangements

CHEM 2124 – Quick Review

E1Bases can be strong or weakPromoted by protic solventsPromoted by AgNO3 if halide substrate3 > 2 > 11st order kineticsNo special geometry requiredRearrangements common - e.g. hydride

and methyl shifts - to get more stable carbocation intermediate

Exothermic reactions Less stable reactants more stable products


Ea

Endothermic reactions More stable reactants less stable products


Ea

Multi-step reactions Reactants reactive intermediates products


CHEM 2124 – Quick Review Reactive Intermediates

Carbocations 3 > 2 > 1 > methyl

Radicals Have an unpaired electron, 3 > 2 > 1 > methyl

Carbenes sp2 hybridized C with lone pair

N N. .

N N+

C H

H:

_

diazomethane

heatC

H

H: + : :

Reactive intermediates continued…Carbanions – negative charge on C

H3C: > 1 > 2 > 3

Relative stabilities of reactive intermediates determined by inductive effects.

Reactive intermediates are generally stabilized by resonance effects.


Hybridizationsp3 – bond angles ~109.5sp2 – bond angles ~120sp – bond angles 180

GeometriesTetrahedral (sp3)Trigonal planar (sp2)Linear (sp)



Formal Charges

Formal charge = group # 1/2 bonding e e’s in lone pairs

Quick method: Group # - # of marks

N N. . +

C H

H:

_

Conformation vs. ConfigurationConformation or shape of a large molecule

changes as a result of rotations about single (sigma) bonds.

Conformational change = change in shapeConfiguration – fixed arrangement of atoms

in space. (cis/trans, R/S, E/Z) Stereoisomers have a fixed configuration.

Configurations cannot change without breaking bonds.

Steric hindrance can affect stabilities of different conformations or configurations.



All isomers

Constitutional or Structural isomers

Stereoisomers

Diastereomers(non-mirror images)

Enantiomers(mirror images)

cis-trans isomers(geometric isomers)

Other diastereomers

R and S designationsCahn-Ingold-Prelog system

Based on assigning priorities to groups or atoms attached to chiral centers (usually C).

Priority based on atomic mass Priority based on first atom of difference

Assignment of R or S designation is made based on following priority 1,2,3 groups around clockwise (R) or counterclockwise (S) when group 4 (lowest priority – commonly H) is in a back position.

R and S designations are absolute configurations.


Relative configurations are assigned experimentally based on rotation of plane-polarized light (+/- or d/l).

Racemic mixture – a 50/50 mixture of +/- (or R/S) isomers.

Racemization – conversion of a formally optically active reactant to a racemic mixture.


E and Z designationsSimilar to Cahn-Ingold-Prelog system for R,S

Based on assigning priorities to groups or atoms attached to C of C=C.

Priority based on atomic mass Priority based on first atom of difference

Assignment of E or Z designation is made based on whether the highest priority groups on each C of C=C are opposite sides of C=C (E for Entgegen) or on the same side of C=C (Z for Zusammen).

E for Epposite - Z for Zame side


Mechanisms – Example 1: Hydrohalogenation of an

alkene

CH3-CH=CH2 + H-Br

Issues to consider:1) Which electrons attack which atoms?2) Regioselectivity – atoms added have a prefered orientation. (Markovnikov vs. anti-Markovnikov)3) Stereochemistry?


Mechanisms continued – Example 2: Dehydrohalogenation

CH3CHBrCH2CH3 + OH

Zaitsev (Saytseff) product vs. Hofmann product


Stabilities of alkenes

Tetrasubstituted > tri > di > mono > un


Trans > geminal > cis

Stoichiometry is still important!!Determining limiting reactantsCalculating maximum or theoretical yieldsCalculating percentage yields

Limiting reactant – the reactant that determines the maximum yield possible. Used up first in the reaction and therefore limits the amount of product that can be made.


CHEM 2124 – Quick Review Limiting reactant + maximum yieldEx: If you mix 0.100 g of acetone with

0.330 g of benzaldehyde in the presence of base in the following reaction, what is the maximum yield of dibenzalacetone.

CHO

CH

CH

CH

CH

O

2 + CH3CCH3

O NaOH

benzaldehyde

MW 106.13

acetone

MW 58.08

dibenzalacetone

MW 234.30

Percentage yield

Amount recovered Theoretical yield

Ex: If you recover 37 mg of product from a reaction with a theoretical yield of 0.106 g what is the percentage yield?

If the lab manual says you should expect to recover 50 mg of product in this reaction, what does this mean?


X 100

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CHEM 2124 – Quick Review