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102 terms

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