CHEM 51A FINAL review cards

Prof Vanderwal 2025

Resonance effect / conjugation

• for C=O IR absorption goes down by 25-30cm-1 for conjugation to alkene alkyne and benzene

• For the bond to be conjugated, they have to be separated by one sigma bond only

Resonance Effect Esters + Amides

Minor Contributors weaken C=O bond

S- charachter

increases sp3=25% —> sp2 = 33% —> sp = 50%

when is entropy not a large contributor to the overall free energy of a rxn

Dihedral Angles for gauche, eclipsed, anti

Anti

180°

Lowest

Gauche

60°

Medium

Partially Eclipsed

120° / 240°

High

Fully Eclipsed

0°

Highest

General trend for chemical shifts

ΔG overall free energy

• Thermodynamically favored = spontaneous = ΔG < 0

• Thermodynamically not favored = ΔG > 0 (positive)

wavenumber trend

• Bond strength: stronger bonds vibrate at higher frequency, so they absorb at higher Wavenumber

• Atom mass: bonds with lighter atoms vibrate at higher frequency, so they absorb at higher Wavenumber  

inductive vs element

element = attached to H

inductive = attached to C

topocity

Nucleophile

“nucleus loving” species that donates an electron pair to form a chemical bond. ( has high electron density)

Nucleophilic sites have

• lone pair 

• pi bond

• negative charge

Electrophile

“electron loving” species that accepts an electron pair to form a chemical bond. (have low electron density)

element effect

looks at the element to which the proton for X-H is attached where X is a main group element. Due to the ability of atom X to attract/hold negative charge

Electronegativity trend ( element effect)

across second row electronegativity increases = more acidic ———>

Atomic radius trend (element effect)

• down a group atomic radius increases = more acidic

• bigger atoms are better @ accommodating -ve charge b/c spread out over large space (Less charge density = more stability)

• acidity increases down a group due to significant differences in bond dissociation energies

  • only true for when the hydrogen (proton) is attached to the heteroatom.

Which element will be removed (deprotonated) first by a base?

• 3 types of 2nd row elements present in the molecule [N,O,C] 

• Electronegative of the atoms holding the charge (O > N > C)

• O-H is most acidic thus gets removed first

Inductive effect (polar effect)

• explains why acidity changes as more polar groups are added

• (ex: adding more “F” to a carboxylic acid increases molecule acidity as they help stabilize negative charge of conjugate base)

• acidity increases when electronegative atoms are added

• the effect is affected by distance ( closer = more acidic)

  • the farther the electronegative group is from the site of the negative charge (or acidic hydrogen), the less it can pull or push electron density.

How does distance affect the inductive effect?

the closer the electronegative atom is to the proton the stronger the acid

Pka trend

Lower Pka = more acidic

Higher Pka = less acidic

resonance effect

• spreads out negative/positive charge over multiple atoms stabilizing the molecule

Acid Base Equilibria Equation

Functional groups

name them

Bronsted Lowry Acid

a species that donates a proton (H+)

Bronsted Lowry Base

a species that accepts a proton (H+)

• must be able to form a bond to a proton

• must contain an available electron pair that can easily be donated to form a new bond. These include lone pairs or electron pairs in π-bonds

curved arrow

shows movement of an electron pair

• The tail of the arrow always begins at an electron pair (such as non-bonding electrons or a covalent bond) and the head points to where that electron pair moves.

Relationship btwn strong acid/base & conjugate

**STRONG ACIDS HAVE WEAK CONJUGATE BASES*

* WEAK ACIDS HAVE STRONG CONJUGATE BASES

amphoteric

act as both acids & bases.

what makes a good resonance 

• more bonds and fewer charges = more stable

• every atom has an octet = more stable

• negative charge on more electronegative atom = more stable

methane

SP3

ALKANES (ex: ethane)

SP3

ALKENES (ex: ethylene) 

SP2

ALKYNE (ex: Acetylene)

SP

Aromatic (ex: BENZENE) 

HALO group (alkyl halide) C-Z σ bond

• short for halogens F,Cl, Br, I

• general structure R - X

• Functional group. -X

HYDROXY ( alcohol)  C-Z σ bond

• functional group -OH

ALOXY (ether)  C-Z σ bond

• functional group -OR

  

AMINO (amine) C-Z σ bond

• functional group -NH2

when does a rxn favored to products vs reactants and the relationship btwn Keq</> 1

• Keq > 1 → Products favored

• Keq < 1 → Reactants favored

• Keq = 1 → Neither side favored (equal stability)

what does more S character mean

the more “s” character a hybrid orbital has, the closer to the nucleus and the more tightly held are its electrons

• sp: 50% s character, linear geometry 180 degrees

• sp²: 33.3% s character, trigonal planar geometry 120 degrees

• sp³: 25% s character, tetrahedral geometry 109.5

polarity

higher electronegativity = greater attraction for electrons

the greater difference in electronegativity the greater the polarity of a bond

why does acidity increase down a column?

b/c positive or negative charge is stabilized when it is spread over a larger volume

• only true for when the hydrogen (proton) is attached to the heteroatom.

  • heteroatom: any atom other than carbon or hydrogen in an organic compound

when does an electron displacement reaction take place?

• If an electron pair is being used to fill an octet of a Lewis acid, then there is no electron pair departing (being displaced).

• If there's only one arrow, there is no electron pair being displaced

Lewis ACID

NONOCTET

chair conformation stability

• if you have one group on the ring the more stable chair will be the one with the group in an equatorial position

• two groups best if they occupy equatorial position 

• if only one can be equatorial in either chair then the more stable will be the one with the larger group in the equatorial position 

wedge and dash

wedged = up

dash = down

*no correlation between up/down and axial/equatorial

newman

• most stable is staggered specifically anti

cis

two groups are up or down

trans

one group is up and the other is down

gauche interactions in in cyclohexane rings

• 2 guache interactions for each axial groups in cyclohexane ring

• 1 guache interaction for each equatorial in cyclohexane ring

methane

1

ethane

2

propane

3

butane

4

pentane

5

hexane

6

heptane

7

octane

8

nonane

9

decane

10

H-H eclipsing

0.9kcal/mol

H-CH3 eclipsing

1.4kcal/mol

dihedral angle

chiral

• objects that exist as enantiomers

• all molecules w/ exactly one stereogenic center are chiral

• a molecule w/ no stereogenic center are usually not chiral

• OPTICALLY ACTIVE

achiral

• molecules contain a plane of symmetry

stereogenic center

• only 3° and 4° carbons can be stereogenic

• any c with 4 diff substituents

• DONT GET FOOLED BY THE RING

drawing enantiomers 

• draw mirror image 

OR

• exchange any 2 substituents dash/wedge

meso compounds 

• molecules w/ stereogenic centers; have a plane of symmetry & mirror images are identical 

• mus thave at least two stereogenic centers

racemic mixture

• an equal amount of two enantiomers 

• optically inactive 

• no rotation (=0°) is observed b/c two enantiomers rotate PPL to an equal extent in opp directions 

roation of polarized clockwise vs counterclockwise

• clockwise = d or +

• counterclockwise = l or -

• two enantiomers rotate plane polarized light to an equal extent but in opposite directions

physical properties of isomers

• Constitutional isomers and diastereomers have different physical properties.

• enantiomers have the same physical properties 

  • can not be separated by physical properties

enantiomeric excess