ochem exam 2

here we are again... ch4-ch7

unsaturated hydrocarbon

contains at least one carbon double or triple bond

diastereoisomers

two molecules which have the same molecular formula and connectivity but are not mirror images of each other and are non-superimposable

formula for alkenes

C_nH_2n

formula for alkynes

C_nH_2n-2

arenes

compounds containing one or more benzene rings

alkene bond structure

120 degree bond angle, 1 sigma and 1 pi bond, shows cis-trans isomerism

cis trans isomers

isomers that have the same order of attachment but different arrangement of atoms in space due to either a ring or a C=C bond

which more stable: cis alkene or trans alkene

trans alkene because of steric strain

alkyne bond structure

linear molecule, 1 sigma and 2 pi bonds

EZ configuration

1) assign priority 2) draw line through center parallel to double bond

if higher priority groups are on the same side, Z

if higher priority groups are on the opposite side, E

priority rules

higher atomic number, higher priority,

priority is assigned at the point of difference

double or triple bonds can be imagined as multiple single bonds

formula for possible number of cis-trans isomers given an alkene with n double bonds

2ⁿ

which is more acidic: alkenes or alkynes

alkyne hydrogen atom is more acidic

hydrohalogenation reaction

alkene + HX

X = Cl, Br, I, etc

hydration reaction

alkene + H₂O

halogenation reaction

alkene + X₂

X = CL, Br, I, etc

hydroboration reaction

alkene + BH₃

hydrogenation reaction

alkene + H₂

transition state indication on an energy diagram

a peak in energy

reaction intermediate indication on an energy diagram

energy minimums between two transition states

activation energy

the difference in energy between the reactants and the transition state

determines the rate of reaction

rate determining step

the step in a reaction sequence that crosses the highest energy barrier

protonation

adding a proton (H) to something

similarly, deprotonation is removing a proton from something

electrophile

wants electrons (+ charge) so it can form a new covalent bond

nucelophile

something with enough electrons (- charge) that can donate a pair to form a new covalent bond

regioselective reactions

favor producing one regioisomer (constitutional isomer) over the other for stability reasons

Markovnikov’s rule

in additions of HX to a C=C bond, the H adds to the carbon with the greater number of hydrogens already on it

carbocation stability ranking

3° > 2° > 1° > methyl

stereoisomer

same molecular formula but different arrangement in 3D space

stereoselective reaction

a reaction where one stereoisomer is formed/destroyed in preference to all others

as stability increases what happens to activation energy

it decreases

why are 3° carbons more stable than 2° carbons

the inductive effect

when does rearrangement occur

it will occur if it will lead to a more stable carbocation

when does rearrangement occur

before nucleophile attacks

halonium ion

an ion consisting of a halogen with a poiitive charge

bromination only produces which type of isomer

trans isomer

what results from hydroboration followed by oxidation of an alkene

hydration of the C=C bond

hydroboration regioselectivity

H adds to the more substituted carbon and boron adds to the less substituted carbon

hydroboration stereoselectivity

boron and H add to the same face of the double bond

catalyst for the reduction of alkenes

a transition metal (most commonly Pd, Pt, or Ni)

heat of hydration relationship with stability

more stable double bond means lower heat of hydration

alkylation

adding an alkyl group to the original alkyneli

alkyne + H₂ (transition metal catalyst) →

alkane

lindlar catalyst

helps you stop a reduction so you go from alkyne → alkene instead of alkyne → alkane

produces cis alkene

syn stereoselective

constitutional isomers

different connectivity of atoms

stereoisomers

same molecular formula and connectivity of atoms but different atom orientation in space

enantiomers

non superposable mirror images

diastereomers

not mirror images

chiral

objects that are not superimposable on their mirror images te

testing chirality around a carbon

chiral if it has 4 unique substituents bonded to it

achiral

objects that are superimposable on their mirror images

stereocenter

an atom at which the interchange of two atoms or groups of atoms bonded to it produces a different stereoisomer

R configuration

clockwise rotation when moving from highest to lowest priorities

S configuration

counterclockwise rotation when moving from highest to lowest priorities

formula for calculating the number of stereoisomers

2ⁿ, where n is the number of stereocenters

meso compound

an achiral compound possessing two or more stereocenters

dextrorotary

clockwise rotation of the plane of plane-polarized light

levorotatory

counterclockwise rotation of the plane of plane-polarized light

specific rotation

the observed rotation of an optically active substance at a concentration of 1g/100 mL in a sample tube 10 cm long

racemic mixture

an equimolar mixture of two enantiomers

haloalkanes

compounds containing a halogen atom covalently bonded to an sp³ hybridized carbon

given the symbol R, R = F, Cl, Br, I, etc

S_N2 mechanism general info

rate = k[haloalkane][nucleophile]

happens in one step, no intermediate

nucleophile attacks from the side opposite the leaving group, new group is put in and old one seperates

S_N1 mechanism general info

rate = k[haloalkane]

first step is rate determining

two or three steps, one intermediate

bond is broken, carbocation and separate ion forms, nucleophile attacks electrophile, proton is removed

product of S_N1 reaction at a stereocenter

racemic mixture

what governs S_N1 reactions

electronic factors (relative stabilities of carbocation intermediates)

what governs S_N2 reactions

steric factors (relative ease of approach of the nucleophile to the site of reaction)

what are the best leaving groups

halogens, conjugate bases of strong acids

what are poor leaving group examples

OH^- and CH₃O^-, or other conjugate bases of weak acids

which S mechanism favors methyl and primary haloalkanes

S_N2

which S mechanism favors weak electrophiles

S_N1

which S mechanism favors strong electrophiles and strong nucleophiles

S_N2

which S mechanism favors secondary and tertiary haloalkanes

S_N1

which S mechanism favors an aprotic solvent

S_N2

E2 mechanism general info

rate = k[base][electrophile]

one step, no intermediates

requires a strong base

E1 mechanism general info

rate = k[electrophile]

two or more steps

requires a weak base

ionization of C-X, carbocation intermediate and halide ion form, proton transfer from carbocation to base

which E mechanism favors primary haloalkanes

E2

which E mechanism favors secondary and tertiary haloalkanes

E1 if weak base is present

E2 if strong base is present

traits of a good nucleophile

negative charge

not bulky

more electronegative

which reaction mechanism goes against Markanokov’s rule

hydroboration oxidation