O Chem Flashcards
Molecular Bonds
Longer bonds are weaker bonds
Triple > Double > Single
Sigma bonds > Pi bonds
sp3 Hybrid Orbitals
s+p+p+p
109.5 degree angle
Tetrahedral, Bent (2 lone pairs), Triganal Plane (one lone pair)
4 sigma bonds
sp2 Hybrid Orbitals
s+p+p
120 degree angle
Triganal planar
sp Hybrid Orbitals
s+p
180 degree angle
Linear
Acid Strength by Orbital
The more s character in a hybrid orbital, the stronger the acid
sp > sp2 > sp3
Resonance
e- density is shifted through regions of a molecule via pi bonds
Resonance reduces basicity (makes an atom more acidic)
Rules:
1) Atoms should have filled orbitals
2) Best structure minimizes formal charges
3) (-) charge is best placed on most electronegative atom
4) (+) charge is best placed on least electronegative atom
Inductive Effect
Induces charge separation in a molecule b/c of delocalization of e- by electronegative atom
- Transfer of e- through sigma bond
- Dissipates over distance
Steric Hindrance
Occurs when two atoms attempt to be in same place at same time
Large substituents should be equitorial on cyclohexane
Aromaticity
Huckel Rule: 4n + 2 (n is integer)
Aromaticity increase stability
Bronsted Lowry Acid-Base
Transfer of H+ from acid to a base
BL acid - compound w/ a H+ that can be lost
BL base - e- available to bind to an H+
As an acid gets stronger it has a weaker conjugate base
Lewis Acid-Base
Transfer of e- pair from base to acid
Lewis bases are nucleophiles
Stength of an acid depends on intramolecular forces
-Acid is stronger w/ an e- withdrawing group
Hydrogen Bonding
Weak bond
Bond btw lone pair of e- and hydrogen w/ partial (+) charge
No H bonding on H’s bonded to carbons
Van der Waals Forces
Exist btw all compounds
Considered only when no other factors
btw temporary dipoles
Constitutional Isomer
Same formula, different connectivity
Finite # of constitutional isomers for a formula
Cycloalkanes
Farther from 109.5 degree angle, the greater the strain and reactivity
Index of Hydrogen Deficiency
Alkanes - C(n)H(2n+2)
IHD = 2(#C) + (#N) - (#H) - (#X) +2 /2
IR Spectroscopy
Finds functional groups OH - broad at 3600 NH - 3400-3200 (medium) Ketone - ~1700 (strong) Aldehyde - ~1740 (strong) Aromatic - 1600-1400
NMR
Takes advantage of hydrogen nucleus magnetic spin
Determines unique hydrogens
Meso Compounds
Have opposing chiral centers, therefore optically inactive
Contain mirror plane through an even # of chiral centers
Stereoisomerism
# of stereoisomers = 2^n -less if some are meso
Nucleophilic Substitution
Substitution of one functional group for another
Nucleophile attacks the electrophile
Sn1 or Sn2
Nucleophile
Smaller nucleophiles are better
Nuc strength is correlated to base strength
Therefore small strong bases are good nucleophiles (but OH and OR can induce elimination)
Electrophile
Strength is correlated to stability of LG
Good LG is stable in solution
Good LG has weak bond w/ carbon
I > Br > Cl»_space; F
Sn2 Reaction
Nuc attacks backside and pushed LG out Causes inversion at chiral carbon One step reaction rate = k[Nuc][Elec] Sn2 favors primary > secondary > tertiary Doesn't form a racemic mixture
Sn1 Reaction
LG leaves before Nuc attacks Does not depend on Nuc concentration Forms carbocation intermediate Favored tertiary > secondary > primary (no methyl) Favored in poor nucleophile rate = k[Elec] Slow two step reaction Forms a racemic mixture Carbocations can rearrange to form tertiary
Free Radical Halogenation
Halogen bond is cleaved to form halogen radicals
Hydrocarbon radical stability: tertiary > secondary > primary > methyl
Mech: 1) initiation; 2) propagation; 3) termination
Selective forming tertiary alkyl halogens (favors substituted product)
E2 Reaction
Carried out under basic conditions at high temperature
Occurs in one step
Requires a bulky base; prevents nuc addition
Substituents must be anti
Yields most substituted alkene
E1 Reaction
Carried out under acidic condition at high temp
Forms a carbocation intermediate (therefore possible rearrangement)
Hofmann Elimination
Forms least substituted alkene
Uses a quaternary amine
Alcohols
Higher boiling points and miscibility in water
Become less hydrophilic as carbons are added
As branching increases, boiling point decreases
As molecular mass increases, boiling point increases
Not good as Nuc, must be deprotonated to an alkoxide
Aldehydes and Ketones
Don't form H-bonds Only slightly miscible in water Reactivity: at electrophilic carbon Ald: IR - 1720-1740; NMR - 9-10 Ket: IR - 1710-1725; NMR - 2.0-2.5
Carbonyl Reactivity
Attack at carbonyl group
C=O, carbon is good electrophile
Reactivity:
Acid halide > Acid anhydride > ester > amide
Aldol Condensation
Kinetic Product - major product minimizes steric hindrance of TS
-chooses least substituted alpha carbon (low T, bulky base)
Thermo Product - maximizes stability of intermediate or product
-choose more substituted alpha carbon (high T, small base)
Epimers
Vary in chirality at most oxidized C
Sugar diastereomers at one chiral center
Anomers
Vary in chirality of least oxidized C
Diastereomers of the cyclic form (alpha or beta)
Beta - Up; alpha - down
Osazome Test
Epimers yield the same osazome
Helps determine identity of sugar
Tollens Test
Reduction of silver by oxidizable sugar
(+) test if silver mirror forms quickly, indicative of an aldose
Amine Basicity
Secondary > Primary > Tertiary > NH3
Amines are increasingly nucleophilic w/ more alkyl groups
Therefore primary amines are hard to synthesize
Distillation
Removes a liquid from another liquid by explooiting differences in boiling point
Fractional Distillation is used when liquids have small boiling point differences
Thin Layer Chromatography
Spot sample on silica gel
Solvent moves up plate interacting w/ samples
Use Rf values for spots of the compounds
-High Rf indicates most solubility in solvent (most like solvent)
Column Chromatography
Separate bulk quantities of product
If a compound travels quickly, then minimal attraction to stationary phase
Gas Chromatography
Vaporize sample into gas phase
Heavier gases move slower and have longer elution times
-b/c heavy and polar gases have higher point and also have longer elution times (therefore lower boiling point compounds come off GC first)
Extraction
Works on solubility
-Takes advantages of drastic solubility differences
Acid-Base:
Separate components on pH
-Loss of charge favors organic (uncharged polar); gain of charge favors water (protonation)
-Brine solution is used to increase differences in aqueous and organic layers
Recrystallization
Solid product separated from solid impurities through selective precipitation
Dissolve solid into hot solvent, filtering out insolubles, then cool solution to precipitate purified crystals
Use minimal solvent so soluble at high temps and insoluble at low temps
Mixed Melting Temperature
A mixture of compounds has a broader melting point indicating impurities
