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topic 6d: halogenoalkanes Flashcards

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1
Q

primary halogenoalkane

A

one carbona ttached to the carbon atom bonded to the halogen

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2
Q

secondary ha

A

2 carbons attached to the carbon atom bonded to the halogen

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3
Q

tertiary ha

A

3 carbons attached to the carbon atom bonded the halogen

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4
Q

waht reactions can HA undergo

A

elimination
substitution

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5
Q

nucleophile

A
  • electron pair donator
  • has a lone pair
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6
Q

type of substitution

A

nucelophilic substitution

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7
Q

what happbesin substiution

A

swapping a halogen atom for another atom/group of atoms

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8
Q

rate of nucleophilic substitution depends on…

A

strength of C-X BOND
- the weaker the bond, the easier it is to break, the faster the reaction

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9
Q

halogenoalkane + KOH (aqueous)

A
  • aquous (water+ethanol COSOLVENT)
  • KOH (or NaOH)
  • heat under reflux
  • produce alcohols
  • :OH- nucleophile
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10
Q

stregnth of oh- nueclophile v water

A

OH- stronger Nu than water as has a full negative charge so more strongly attracted to delta + c

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11
Q

halogenoalkane + silver nitrate

A
  • aqueous silver nitrate
  • in WATER AND ETHANOL (cosolvents)
  • warm water nucleophile
  • forms alcohol
  • hal - reacts with ag+ to form ppt
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12
Q

ha + potassium cyanide

A
  • nitriles
  • cyanide ion is nucleophile
  • INCREASES LENGTH OF CARBON CHASIN
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13
Q

ha + ammonia

A

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

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14
Q

ha + KOH (ehtanolic)

A
  • alkenes
    -only ethanol
  • heat under reflux
  • OH- acts as a base
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15
Q

hydrolysis

A

splitting of a molecule by reaction with water

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16
Q

water as a nucleophile

A

POOR
- reacts slowly with HA in nucleophilic substition reaction

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17
Q

how to compare rate of hydrolysis

A
  • add ethanol
  • add silver nitrate
  • water bath
  • compare rate of formation of precipitate
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18
Q

AgI colour + speed

A
  • yellow ppt
  • fastest speed
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19
Q

AgBr colour and speed

A
  • cream ppt
  • mid speed
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20
Q

AgCl colour and speed

A
  • white ppt
  • slowest
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21
Q

primary HA nu sub mechanism

A

SN2

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22
Q

tertiary HA nu sub mechanism

A

SN1

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23
Q

why tertiary do SN1

A
  • tertairy CARBOCATION more stable
  • more alkyl groups causing positive inductive effect
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24
Q

why primary dont do sn1

A
  • primary carbocation unstable
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25
why primary do SN2 but tertiary dont
- nuclephle must attack at opposite side to leaving group - too much steric hindrance w tertiary
26
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
27
all C-X bonds...
are polar C-F most polar
28
solublility of HA
- insoluble in water HA have large R groups non polar SOLUBLE IN HYDROCARBON
29
BOILING POINT
- increases down group - eg R-I has highest - size of atom (LFs) outweighs polarity
30
reactivty of ha depends on
strength of C-X bond
31
how does the hydrolysis work
- C-X breaks - X- ion released in solution - reacts w Ag+ to form AgX solid
32
CH3Cl + OH (aqeuous)
CH3OH + Cl-
33
CH3Cl+ CN-
CH3CN + Cl-
34
CH3Cl + 2NH3
CH3NH2 + NH4Cl
35
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
36
SN1 exo or endo
- exothermic - energy profile = reactants low, intermediate high, products very low
37
chiral defintion
- 4 different groups attached to carbon - non superimposable on its mirror image
38
2 types of STEREO isomerism
1. geometric (z,e) 2. optical (chirality)
39
C-X bond fission
heterolytically forms ions
40
bond formed in substitution
DATIVE bond between nucleophile and carbon halide is leaving group
41
sn2 named beacuse
-2nd order overall - 2 species in RDS
42
sn1 named becuase
-1st order overall - 1 species in RDS
43
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
44
sN2 energy profile
- exo - rectants higher than products
45
need for ethanol + water cosolvent
- used with KOH and silver nitrate - water dissolves the other thing, ethanol dissolves halogenoalkane
46
cyanide ion dot and cross
c triple bond n new elecrton on c - LP ON C IS MORE REACTIVE THAN THAT ON N
47
ha + cyanide product
alkane nitrile eg butane nitrile
48
sn2 is the one with the ...
HALF BONDS
49
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
50
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
51
why do u get oversubtituition with ammonia
- lone pair of electrons on the nitrogen in the organic product
52
what happens with xs halogenoalkane in the ha + ammonia
larger proprotion of oversubstitution so more quaternary ammonium salt
53
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
54
nh3 CONDITIONS
- concentrated nh3 - ethanolic - head in sealed vessel under high pressure
55
waht to remember about product of sn1
reverse the wedge and dashed line
56
hoew to test for halogenoalkane
- mix w ethanol + water cosolvent - add agno3 and warm in water bath - water nucleophile - forms alcohol + X- which reacts w Ag+ - ppt formed
57
alterntive testing for halogeolakne
- add KOH - nitric acid (prevents formation of silver oxide) - silver nitrate
58
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
59
SN2 products polarimeter
- if reactant was a single enantiomer, product is the other enantiomer. -in polarineter, rotation of light (optical activity) reversed
60
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
61
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
62
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
63
2 ENANTIOMERS HOW TO DISTINGUISH
- rotate the plane of plane polarised light - equally but in opposite directions
64
SN1 POLARIMETER
- no effect - racemic mixture - CARBOCATION TRIGONAL PLANAR INTERMEDIATE - equal prob of attack of nucleophile above and below the pla e
65
stereoisomers
- same structural formula - different spatial arangement
66
role of water in nucleohpilic substitution (2)
- nucleophile - bonds to carbocation
67
role of ater in elimination
- acts as a base - removes a proton
68
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
69
in the elimintation mechanism using ethanolic KOH, what is the role of the oh-
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