OChem Class 1 Flashcards
Saturated Alkanes
All carbons have maximum number of H atoms attached
CnH2n+2
Degree of Unsaturation
H2 molecule is removed
DOU = (2n+2) - x /2
x = # of hydrogen atoms n = # of carbon atoms
Halogen acts like hydrogen atoms
O2 does nothing
Nitrogen has to be subtracted from the total of hydrogen atoms
Properties of Unstable vs Stable molecule
Unstable:
- high reactivity with environment
- shorter lifespan
- high in energy
Stable:
- Low reactivity with environment
- longer lifespan
- low in energy
Ring strain & strain energy
Ideal sp3 bond angle is 109.5, if it differs from this, it’ll cause ring strain
3 membered ring has more strain than 4 5 membered ring has minimal 6 =0 7 + = acts similar to 5 membered ring 14 = 0
What does ring strain do?
Destabilizes ring, weakens C-C bond, increases reactivity
What molecule can relieve ring strain?
H2 as a catalyst (usually in alkenes & alkyenes, not alkanes)
Electropositive vs Electronegative substituents
Electropositive - Donate e- density (eg. alkyl groups)
Electronegative - Withdraw e- density (eg. O, halogens)
Induction
Electrons in a sigma bond shift toward the more electronegative atom
FONClBrSCH
Carbocation stability
tertiary > secondary > primary > methyl
Carboanion stability
tertiary < secondary < primary < methyl
Resonance stabilization
Occurs due to delocalization of electrons
Electrons travel from nucleophillic (high negative charge) to electrophillic (low negative charge)
Resonance rules
- Octet satisfied for ALL atoms
- Least formal charge
- Negative charge on the more electronegative atom
Bronsted-Lowry Acid
Donates H+ and leaves behind conjugate base
The more stable conjugate base is, the strong the acid
3 factors that influence conjugate base stability
- Electronegativity- A more EN atom attached to H can handle negative charge better
- Resonance - Increases stability of anionic conjugate base
- Induction - proximity & electronegativity of EWG
Strong acids to weak acids list
HBr, HI, HCl, HNO3, H2SO4, HClO4 > Sulfonic acid > carboxylic acid > phenol > alcohol & water > ketone & aldehyde > sp hybrid > sp2 hybrid > sp3 hybrid
Nucleophile vs Electrophile
Electrophile is electron loving, accepts e- pair
Nucleophile is nucleus loving, donates e- pair
Lewis base vs acid
Lewis base donates e-
Lewis acid accepts e-
Nucleophilicity trend
Increases right to left, top to bottom
Leaving Groups
- Parallel acidity (good leaving group = good acid)
- A good LG means it is a weak base of a strong acid, resonance stabilized or neutral LG. (water)
What makes a molecule more stable?
Resonance
Induction
Size
Electronegativity
What does reactivity depend on?
Leaving group ability
Nucleophile strength
Acid strength
Isomer
Same molecular formula with different compounds
Constitutional vs Conformational Isomer
Constitutional (structural)
- different atomic connectivity thus different chemical & physical properties
Conformational
- same atomic connectivity & identical chemical & physical properties
Newman projections for n-butane
If methyl groups are 180° apart it is ANTI, and if H groups are 60° apart it is staggered (most stable)
If methyl groups are 60° apart it’s GAUCHE (intermediate stable)
If methyl groups are 0° apart (eclipsed) it is SYN (least stable)
When drawing chair conformations
- Axial alternates up and down
- Equitorial alternates opposite to that
- When you have large substituent, better to be equitorial than axial because of unfavourable steric interactions
Trans vs cis
Trans - both groups facing opposite direction
Cis - both groups facing same direction
Chirality
Not superimposable on its mirror image
No plane of symmetry
How to locate a chiral center
- sp3 hybridization
- tetrahedral geometry
- 4 different substituents
- carbons part of pi bond (double bond) won’t have chiral center because it’ll only have 3 groups NOT 4
How to calculate number of isomers knowing how many chiral centers
2^n
n = # of chiral centers
Optical Acitivty
Chiral molecules rotate plane-polarized light & are optically active
d = dextrorotary (clockwise) +
l = levorotary (counterclockwise) -
+/- –> optical rotation
d/l –> relative sterochemistry
Assigning priority to atoms bonded to chiral center
- Atom with highest atomic number has highest priority
- Heavier isotopes have higher priority
- if two atoms on a chiral center are identical, move to the next atom to find the first point of difference; the atom with the highest atomic number takes priority
- Bonded atoms count as an equivalent number of single bonded atoms
Absolute configuration (R & S)
- Assign priority (1-4) to atoms directly bonded to chiral center
- Ensure lowest priorty substituent is in the “away” (dashed) position. If not, switch it with the substituent that is taking that place
- Trace arc going from 1-3
- Counter-clockwise = S
- Clockwise = R
* If an exchange happened in step 2, use the reverse letter
Steroisomers
Same connectivity but differ in spatial arrangement
Can be diasteromer, enantiomer or meso compound
Enantiomer
- Non superimposable, mirror images
- Opposite configuration at ALL chiral centers
- Each enantiomer has optical rotation of equal magnitude, but opposite sign
- All other chemical & physical properties are identical
- Process to separate enantiomers is resolution
Meso Compounds
- Must contain chiral centers
- Have internal plane of symmetry
- not optically active (achiral)
Diastereomer
- Non-superimposable, non mirror image
- All chemical & physical properties are different
- Unrelated optical activities
- Opposite configuration for at least ONE but not all chiral centers
Can be either Geometric isomer OR Epimer
Geometric Isomer
Does not need chiral center
Restricted rotation about double bond or ring
All chemical/physical properties are different
Z/E —> Cis/trans
Trans is better because less steric interaction
Epimers
Differ at ONLY ONE chiral center
