CHEM 232 Exam 2

exothermic

∆H of the reaction is negative; thermodynamically favorable; products are more stable

endothermic

∆H of the reaction is positive; thermodynamically unfavorable; starting materials are more stable

homolytic bond cleavage

bond breaking where each atom takes one electron from the shared bond, forming radicals (represented by single-barbed arrows)

heterolytic bond cleavage

bond breaking where one atom takes both electrons from the shared bond, forming ions (represented by standard arrows)

radical reactions

what reactions have homolytic bond cleavage?

acid base, SN2, SN1, E1, E2

what 5 reactions have heterolytic bond cleavage

bond dissociation energy

term for the measure of bond strength and the amount of energy required to homolytically cleave a specific covalent bond

endothermic; starting materials

if K<0, is the reaction exothermic or endothermic and are the SM or products favored?

exothermic; products favored

if K>1, is the reaction exothermic or endothermic and are the SM or products favored?

initiation, propagation, termination

what are the three steps of radical halogenation mechanisms in order?

radical

a highly reactive chemical species with an unpaired electron

initiation

step 1 of radical halogenation where radicals are created from non-radicals through heat or light (think of a spark)

propagation

step 2 of radical halogenation where a radical reacts with a stable molecule to form a product and a new radical (two step cycle) (think of a ongoing fire)

termination

step 3 of radical halogenation where two radical combine to form a stable bond, ending the radical chain and neutralizing the radicals (think of water putting out a fire)

rate determining step

highest energy transition state in a reaction; controls the overall reaction

propagation shuts down

if radicals are removed too early (meaning termination happens too early), what ends up happening?

low radical concentration

how can terminiation be avoided?

transition state

highest-energy, unstable arrangement of atoms at the peak of a reaction coordinate where bonds are partially formed and broken

hammond postulate

statement that the structure and energy of the transition state resembles that of the starting material, intermediate, or product that it is closest to in energy

starting materials

what does an early transition state look like?

products

what does a late transition state look like?

faster

is an early transition state a faster or slower step?

slower

is an late transition state a faster or slower step?

substitution

the number of C’s attached to a central sp³ C

electron delocalization; stabilization

fill in the blanks: more substitution leads to more _______ which leads to more ______

lower

is a higher or lower energy pathway more favored (which one makes the product faster?)

hyperconjugation

the stabilization of a radical (or carbocation) through the donation of electron density from parallel, adjacent σ bonds (like C-H or C-C bonds) into the partially empty p orbital

selectivity

the preference of a reactant to react at a specific site

bromination

what is more highly selective: bromination or chlorination (think about substituted carbons due to an endothermic rate-determining step)

most stable

fill in the blank: in halogenation reactions with Br₂ the ______ radical always produces the major product

electrophile

an electron acceptor (acts like a lewis acid)

nucleophile

an electron donor (acts like a lewis base)

carbon connected to the leaving group

what does a nucleophile attach to?

concerted

bond breaking occurs in one step

SN2

one-step (concerted) reaction where a nucleophile does a backside attack on an electrophile, removing the leaving group and resulting in an inverted structure

rate = k[E-LG][Nu]

what is the rate equation for SN2

retention product

what product is given by a frontside attack? (Nu takes LG spot and other groups don’t move)

inversion product

what product is given by a backside attack? (Nu shoves groups out of the way and the other groups shift into open space)

no, only backside attack

do SN2 mechanisms include both frontside and backside attack? which one?

double inversion

occurs when two consecutive SN2 reactions take place sequentially at the same carbon center; the first SN2 step inverts the stereocenter; the second SN2 step inverts it back

a net retention of configuration

what does double inversion result in?

HOMO

the highest energy orbital containing electrons for SN1 and SN2, typically the lone pair of the nucleophile; focuses on attacking electron pair

LUMO

the lowest energy empty orbital in an SN2 reaction; the σ* antibonding orbital opposite of the leaving group; focuses on broken bond orbitals

more stable

fill in the blank: a _____ lone pair means a better LG

right

leaving group ability increases to the _____ along a row

first row elements

what elements are bad leaving groups?

alpha carbon

carbon attached to the functional group of interest; in SN2, it is the carbon with the LG which is the carbon the nucleophile attacks

beta carbon

carbons attached to the alpha carbon

decreases

what happens to SN2 reaction rates as alpha and beta carbons become more substituted

axial

what positioning does a nucleophile have an easier time doing a backside attack? axial or equitorial

equitorial

what is more stable, equitorial or axial?

greater negative charge, less electronegative, less bulky alpha carbon

what are 3 trends that make a nucleophile more nucleophillic?

nucleophilicity increases down a column

for neutral nucleophiles, what is the trend for nucleophilicity? (hint: think of periodic trends and columns)

polar aprotic solvents

solvents with weak electrostatic interaction with nucleophile; only allows dipole-dipole interactions; smaller atoms are more basic and nucleophillic (up the column)

polar protic solvents

solvents with strong electrostatic interactions with ions; allows for hydrogen bonding that forms cages around small ions; larger, charge delocalized atoms are more nucleophillic (down the column)

rate = k[E-LG]

what is the rate equation for SN1?

electrophile bonded to leaving group

what is active in the RDS for SN1 reactions? (Hint: only one thing)

frontside and backside attack

what two attacks are in an SN1 mechanism?

LG leaves, bond breaks, RDS forms

what occurs in step 1 of SN1 mechanism (dissociation)

nucleophile attacks, bond is made

what occurs in step 2 of SN1 mechanism (coordination)

proton transfer (deprotonation)

what occurs in step 3 of SN1 mechanism to neutralize charges (hint: acid/base step)

no

does the nucleophile impact an SN1 reaction rate?

resonance/conjugation

what is the strongest form of electron delocalization effect on carbocations?

destabilizes

does inductive effect stabilize or destabilize carbocations?

as the carbocation decreases in energy, so does the transition state

what is the correlation between a carbocation and the transition state?

polar protic

what solvents have strong electrostatic interactions with carbocations that stabilize the carbocation and increase reaction rate

good LG and strong Nu

what are two requirements for SN2 reactions (think of LG and Nu)

polar aprotic

the maximum reaction rate for an SN2 mechanism is through which solvent?

methyl > primary > secondary

what is the order of substituted carbons is preferred for SN2 mechanisms?

carbocation

what is the intermediate for an SN1 reaction?

2

how many stereoisomers can SN1 mechanisms form?

good LG and stable carbocation

what are the 2 requirements for SN1 mechanisms (think of LG and carbocation)

more substitutions, tertiary > secondary

what level of substitution is preferred for SN1 mechanisms?

polar protic

the maximum reaction rate for an SN1 mechanism is through which solvent?