topic 6d: halogenoalkanes Flashcards
primary halogenoalkane
one carbona ttached to the carbon atom bonded to the halogen
secondary ha
2 carbons attached to the carbon atom bonded to the halogen
tertiary ha
3 carbons attached to the carbon atom bonded the halogen
waht reactions can HA undergo
elimination
substitution
nucleophile
- electron pair donator
- has a lone pair
type of substitution
nucelophilic substitution
what happbesin substiution
swapping a halogen atom for another atom/group of atoms
rate of nucleophilic substitution depends on…
strength of C-X BOND
- the weaker the bond, the easier it is to break, the faster the reaction
halogenoalkane + KOH (aqueous)
- aquous (water+ethanol COSOLVENT)
- KOH (or NaOH)
- heat under reflux
- produce alcohols
- :OH- nucleophile
stregnth of oh- nueclophile v water
OH- stronger Nu than water as has a full negative charge so more strongly attracted to delta + c
halogenoalkane + silver nitrate
- aqueous silver nitrate
- in WATER AND ETHANOL (cosolvents)
- warm water nucleophile
- forms alcohol
- hal - reacts with ag+ to form ppt
ha + potassium cyanide
- nitriles
- cyanide ion is nucleophile
- INCREASES LENGTH OF CARBON CHASIN
ha + ammonia
PRIMARY AMINS
- amonia dissolved in ethanaol
- HEAT IN A SEALED VESSEL (under pressure)
- ammonia nucleophile
- meth/eth/prop YL AMINE
- prevent over sub using excess ammonia
ha + KOH (ehtanolic)
- alkenes
-only ethanol - heat under reflux
- OH- acts as a base
hydrolysis
splitting of a molecule by reaction with water
water as a nucleophile
POOR
- reacts slowly with HA in nucleophilic substition reaction
how to compare rate of hydrolysis
- add ethanol
- add silver nitrate
- compare rate of formation of precipitate
AgI colour + speed
- yellow ppt
- fastest speed
AgBr colour and speed
- cream ppt
- mid speed
AgCl colour and speed
- white ppt
- slowest
primary HA nu sub mechanism
SN2
tertiary HA nu sub mechanism
SN1
why tertiary do SN1
- tertairy CARBOCATION more stable
- more alkyl groups causing positive inductive effect
why primary dont do sn1
- primary carbocation unstable
why primary do SN1 but tertiary dont
- nuclephle must attack at opposite side to leaving group
- too much steric hindrance w tertiary
ha + KOH diff types of reaction
- AQUOUS = substiuttion to form alochol
- ETHANOLIC = elimination to form alkene
- usually both happen
- primary tend to wards sub, tertiary towards elimination
all C-X bonds…
are polar
C-F most polar
solublility of HA
- insoluble in water
HA have large R groups non polar
SOLUBLE IN HYDROCARBON
BOILING POINT
- increases down group
- eg R-I has highest
- size of atom (LFs) outweighs polarity
reactivty of ha depends on
strength of C-X bond
how does the hydrolysis work
- C-X breaks
- X- ion released in solution
- reacts w Ag+ to form AgX solid
CH3Cl + OH (aqeuous)
CH3OH + Cl-
CH3Cl+ CN-
CH3CN + Cl-
CH3Cl + 2NH3
CH3NH2 + NH4Cl
describe SN1
- C-X bond spontaneously breaks (SLOW AS RDS)
- forms the carbocation intermediate. draw nucleophile attacking a C-H bond
- final step with full product + halide ion
SN1 exo or endo
- exothermic
- energy profile = reactants low, intermediate high, products very low
chiral defintion
- 4 different groups attached to carbon
- non superimposable on its mirror image
2 types of STEREO isomerism
- geometric (z,e)
- optical (chirality)
C-X bond fission
heterolytically
forms ions
bond formed in substitution
DATIVE bond between nucleophile and carbon
halide is leaving group
sn2 named beacuse
-2nd order overall
- 2 species in RDS
sn1 named becuase
-1st order overall
- 1 species in RDS
SN2 describe mechanism (draw)
- 3d arrangement
- c-x bond breaks to the x. simultaneoulsy lone pair from nucleophile goes to central c
-STEP2: TRANSITION STATE: partial bonds form , one w nucleophile and one with halogen. overall charge if nucleophile was charged - final product 3d structure + halide
sN2 energy profile
- exo
- rectants higher than products
need for ethanol + water cosolvent
- used with KOH and silver nitrate
- water dissolves the other thing, ethanol dissolves halogenoalkane
cyanide ion dot and cross
c triple bond n
new elecrton on c
- LP ON C IS MORE REACTIVE THAN THAT ON N
ha + cyanide product
alkane nitrile
eg butane nitrile
sn2 is the one with the …
HALF BONDS
why is the NH3 heat in a sealed vessel under pressure not heat under reflux
- NH3 boiling temp is below that of water
- so wont condense in a water cooled condenser
describe SN2 with the ammonia
-c-x bond breaks, lone pair on nh3 goes to delta + c
- second step with half bonds
- 3rd step; full 3d without halide. n-h breaks so n+, lone pair on another nh3 forms bond with h
- final has nh2 + nh4x
why do u get oversubtituition with ammonia
- lone pair of electrons on the nitrogen in the organic product
what happens with xs halogenoalkane in the ha + ammonia
larger proprotion of oversubstitution so more quaternary ammonium salt
describe the ethanolic potassium hydroxide elimintation mechanism
TAKE ADJACENT CARBON TO C-X
- nucleophile arrow to H
- SAME h-c arrow to c-c
-c-x to x
- forms the alkene AND X- AND H2O
nh3 CONDITIONS
- concentrated nh3
- ethanolic
- head in sealed vessel under high pressure
waht to remember about product of sn1
reverse the wedge and dashed line
hoew to test for halogenoalkane
- mix w ethanol
- add agno3 and warm in water bath
alterntive testing for halogeolakne
- add KOH
- nitric acid (prevents formation of silver oxide)
- silver nitrate
why do primary go by sn2
IF SN1:
- forms a v unstable primary carbocation
ADVANTAGE SN2
- low activation energy barrier as little steric hindrance around delta + carbon, so neuclophiles can easily attack
SN2 products polarimeter
- if reactant was a single enantiomer, product is the other enantiomer.
-in polarineter, rotation of light (optical activity) reversed
SN1 assessing products
- equal propbability of nucleophile attacking above or below plane
- equal conc of each enantiomer made so no overall efefct on rotation of light
why if both are chiral does one NOT rotate lifht
- the first is a single enantiomer
- second is a RACEMIC MIXTURE: equal amounts of both enantiomers
SN2 POLARIMETER
- optical activity, rotated in opposite direction
- ONE ENANTIOMER FORMED
-Nuclophile attacks from the opposite side to the leaving group -> only on ONE SIDE
2 ENANTIOMERS HOW TO DISTINGUISH
- rotate the plane of plane polarised light
- equally but in opposite directions
SN1 POLARIMETER
- no effect
- racemic mixture
- CARBOCATION TRIGONAL PLANAR INTERMEDIATE
- equal prob of attack of nucleophile above and below the pla e
stereoisomers
- same structural formula
- different spatial arangement
role of water in nucleohpilic substitution (2)
- nucleophile
- bonds to carbocation
role of ater in elimination
- acts as a base
- removes a proton
how are ions formed in the halogenoalkane even though theres only covalent bonds (2)
- water hydrolyses haloalkane
- heterolytic bond fission of c-hal to form ions
