Kaplan MCAT OChem Ch. 4: Alkanes, Alkyl Halides, and Substitution Rxns

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saturated hydrocarbons

no DBs

w/ max number of H it can hold

alkanes

most are combustible liquids and gases

longer alkanes

tend to exist as liquids

classify carbons by number of other carbon atoms to which they are directly bonded to

1st, 2nd, 3rd, 4th

phys. property of alkane

as MW inc. => so do MP, BP, and density


heavier molecule -> harder for molecule to break away from other and enter higher-energy gas phase

straight-chain compounds up to 4 carbons (alkane)

in gaseous state

chain of 5 to 16 carbons (alkane)

exist as liquids

longer chain compounds (alkane)

waxes and harder solids

branched molecule alkane

slightly lower BP than straight chain

this reduces SA available for interactions w/ neighboring molecules, weakened intermolecular attactive forces (van der Waals)

more symm. alkane

easier to stack => higher MP

alkanes

very stable molecules, unless we agitate them

type of alkane rxns

combustion, free-radical halogenation, pyrolysis

combustion

is the rxn of alkanes w/ molecular oxygen to form CO2, water, and heat

equation for combustion

http://www.personal.kent.edu/~cearley/gen50/balance/C3H8_balance.png + heat

combustion

one problem is that it's often incomplete => sig. CO => air pollution

internal combustion

N in air often oxidized accidentally => nitrous oxide can inc. engine power of car

halogenation

one or more H atoms are replaced w/ halogen atom (Cl, Br, or I) via free-radical substitutin mech

free-radical substitution

where in halogenation, H atoms are replaced w/ halogen atoms via this mech

halogen

3 steps

2nd step can occur multiple times before final step occurs

free-radical halogenation: initiation

diatomic halogens are homo cleaved (2 electrons of sigma bond are split equally) by either heat or ultraviolet => two free radicals

free radicals

neutral species w/ unpaired electrons

extremely reactive and readily attack alkanes

free-radical halogenation: initiation

http://upload.wikimedia.org/wikipedia/commons/thumb/3/32/MethaneChlorinationMechanismInitiation.svg/260px-MethaneChlorinationMechanismInitiation.svg.png

free-radical halogenation: propagation

radical produces another radical

free-radical halogenation: propagation

1st step: free radical reacts w/ an alkane => removes H atom => form HX, creating an alkyl radical

free-radical halogenation: propagation

2nd step: alkyl radical reacts w/ X2 => alkyl halide => X radical

free-radical halogenation: propagation

begins and ends w/ radical

free-radical halogenation: propagation

http://www.cf.edu/departments/instruction/las/science/chemistry/mechanisms/radical-chlorination/chlorination-propagation-spets.gif

free-radical halogenation: termination

two free radicals combine w/ one another => stable molecule

free-radical halogenation: termination

http://upload.wikimedia.org/wikipedia/commons/thumb/5/5a/MethaneChlorinationTermination.svg/358px-MethaneChlorinationTermination.svg.png

larger alkanes

have many H's for free radical to attack

bromine radicals

react fairly slowly and primarily attack H on C atom that can form most stable free radical (most sub)

more stable intermediate => more likely rxn is to occur

bromine radicals

3 > 2 > 1 > methyl

free radical chlorination

more rapid process

depends on stability of intermediate and number of H's present

free radical chlorination

more likely to replace primary H's if it's the most abudant type, despite instability

free radical chlorination

decreased selectivity

free radical chlorination

yield mixtures of products and useful only when just one type of hydrogen present

pyrolysis

when a molecule broken down by heat

rxn involving UV

almost always see a free radical

rxn involving heat

possibility of free radicals and elimination

cracking

another name of pyrolysis

cracking

most commonly used to reduce the avg. MW of heavy oils and inc. the production of the more desirable volatile compounds

pyrolysis of alkanes

C-C bonds are cleaved => smaller-chain alkyl radicals => can recombine to form a variety of alkes

disproportionation

a radical transfers a H atom to another radical => alkane and an alkene

disproportionation example

http://polymer.w9of.com/images/disproportionation.jpg

Nu sub rxns

alkyl halides and other sub C atoms can take part in these type of rxns

Nu

are e- rich species that are attracted to pos. charged or pos. polarized atoms

electrophiles

pos. charged or pos. polarized atoms

characteristics of Nu

basicity and size and polarizability

Nu basicity

strong the base => more likley to attract a pos. charged proton (Bronsted-Lowry)

Nu and bascity

directly related

Nu strength (basicity)

RO- > HO- > RCO2 - > ROH > H2O

protic solvents

solvents w/ protons in solution, like water and alcohol

in protic solvents

large atoms tend to be better Nu because can shed solvating protons surrounding them and are more polarizable

electrons can shift around => making some more neg than others => better chance to make it to the Electrophile, while still neg. enough to attack

Nu strength (protic solvents)

CN- > I- > RO- > HO- > Br- > Cl- > F- > H20

aprotic solvents

solvents w/o protons

in aprotic solvents

Nu is related to basicity

in aprotic solvents

Nu don't have fancy proton coats surrounding them, they aren't solvated

Nu strength (in aprotic solvents)

F- > Cl- > Br- > I-

in protic solvents

size matters

capable of H bonding, or donating protons

in protic solvents

smaller atoms => easily surrounded by solvent => dec. its ability to act as Nu

leaving groups (Nu)

best ones are those that are weak bases, or stable anions or neutral species => can easily accomodate electron pair

leaving groups (Nu)

good ones are conjugate bases of SA

leaving groups (Nu)

in case of halogens, opposite of base strength

I- > Br- > Cl- > F-

leaving groups (Nu)

weak bases make good these because able to spread electron density => species more neutral or more stable

Sn1 rxns

unimolecular sub rxn

Sn1 rxns

tells us that rate of rxn depends on only one species, the substrate (orignal molecule)

Sn1 rxns

RDS is disso. of this species to form a stable, pos. charged ion called carbocation

carbocation

this pos. charged ion is stable form formed in RDS of Sn1 rxns

strong electrophile => picks up anything that comes near it w/ lone electrons

substrate

original molecule in Sn1 rxns

mech of Sn1

involve 2 steps

mech of Sn1 steps

1. disso. of molecule => carbocation and good LG
2. combo of carbocation w/ a Nu

mech of Sn1

can be accomplished using polar protic solvents w/ lone electron pairs, since electron rich groups can solvate the carbocation and help stabilize

mech of Sn1

carbocations are also stabilized by charge delocalization

more sub => more stable

3 > 2 > 1 > methyl

mech of Sn1

better LG than Nu => otherwise, reverse rxn will outcompete the forward one

mech of Sn1

doesn't require strong Nu (this is in Sn2)

rate of Sn1 rxns

depends on its slowest step (RDS or RL)

rate of Sn1 rxns

slowest step is disso. of the molecule to form the carbocation => energetically unfavorable

rate of Sn1 rxns

only reactant is original molecule (substrate), so this depends only on concentration of original molecule

rate of Sn1 rxns

rate = k[RX], first order rxn

rate of Sn1 rxns

can be increased by anything the accelerates the formation of carbocation

structural factors of Sn1 rxns

highly sub. alkyl halides allow for distribution of pos. charge over a greater number of C atoms => most stable carbocation

solvent effects Sn1

highly polar solvents better at surrounding and isolating

solvation stabilizes the intermediate

nature of leaving group

WB disso. more easily from alkyl chain => better this => inc. rate of carbocation formation

Sn2 rxns

one step

Sn2 rxns

involves strong nucleophile pushing into a compound while displacing LG at same time

Sn2 rxns

involves two molecules

mech of Sn2

involves backside attack

mech of Sn2

Nu must be strong and substrate can't be sterically hindered

mech of Sn2

primary substrates most likely to undergo this, followed by 2ndary

mech of Sn2

Nu attacks reactant backside => trigonal bipyramidal transition state (sp2)

trigonal bipyramidal

transition state (sp2), results from Nu backside attack

rate of sn2 rxn

single step involves two reacting species: substrate (molecule w/ LG , often alkyl halide or a tosylate)

rate of sn2 rxn

2nd order kinetics:

rate = k[Nu][RX]

intermediate

well defined species w/ finite lifetime

must be at relative minimum energy for this to occur

transition state

represents a max (in energy) between two minima on a reaction coordinate

sn1 stereochemistry

carbocation intermediate has three groups bound to it

sn1 stereochemistry

planar shape = carbocation , 120 degrees, achiral

sn1 stereochemistry

Nu can attack either top or bottom => two diff products (depending)

sn1 stereochemistry

if original compound optically => products are racemic mixture => no longer optically active

Sn2 stereochemistry

moelcule will flip => inversion of config

Sn2 stereochemistry

inversion of stereochem => lead to an inversion of abs. configuration only if LG and Nu have same priority (R -> S),

if having diff. priorities (even though molecule will still flip => designation will not be changed

Nu sub rxn

even though alkyl halides are equipped w/ good LG, they can still undergo this

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