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chemistry > module six: organic chemistry and analysis > Flashcards

module six: organic chemistry and analysis Flashcards

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1
Q

kekulé’s model of benzene

A

alternating double and single bonds

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2
Q

delocalised model of benzene

A

delocalised electron ring

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3
Q

main features of delocalised benzene model

A

planar, cyclic, hexagonal hydrocarbon with 6 carbons, 6 hydrogens
carbon uses 3/4 electrons to bond to 2 carbons, 1 hydrogen
1 electron in p-orbital at right angles to plane
p-orbitals overlap sideways above + below plane of carbon atoms, forming a ring of electron density
overlapping creates system of 𝝅-bonds, across all 6 carbons in ring
electrons in 𝝅-bonds are delocalised

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4
Q

molecular and empirical formula of benzene

A

molecular: C6H6
empirical: CH

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5
Q

bond angles in benzene

A

120

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6
Q

evidence that does not support kekulé’s model

A

all carbon bonds are the same length, even though double and single bonds have different lengths
benzene is less reactive than alkenes, benzene does not undergo electrophilic addition or decolourise bromine under normal conditions
enthalpy change of hydrogenation is lower than expected - more stable molecule

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7
Q

technique to find bond lengths of benzene

A

x-ray diffraction

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8
Q

4th electron in the p-orbital of each carbon atom in benzene

A

delocalises to form rings above and below the hexagon
forms rings of delocalised electron density above and below the hexagon

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9
Q

why is benzene attacked by electrophiles

A

high electron density above/below the ring due to the delocalised electrons

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10
Q

type of reaction: nitration of benzene

A

electrophilic substitution

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11
Q

how is the NO2 + ion created

A

conc. H2SO4, conc. HNO3
H2SO4 + HNO3 –> HSO4 - + H2O + NO2 +

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12
Q

how is H2SO4 catalyst regenerated after electrophilic subsitution

A

H+ + HSO4 –> H2SO4
hydrogen ion from benzene

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13
Q

why does benzene need a halogen carrier

A

benzene does not react directly with halogens as aromatic ring is too stable

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14
Q

nitration of benzene

A

reagents: conc. HNO3, conc. H2SO4 (catalyst)
conditions: 50°C - monosubstitution
equation: C6H6 + HNO3 –> C6H5NO2 + H2O

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15
Q

nitration of benzene above 50°C

A

produces dinitrobenzene
below 50°C ensures monosubstitution

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16
Q

nitration of benzene mechanism

A

**step one: ** NO2 + ion produced
step two: electron pair leaves delocalised system to form a bond to NO2
an unstable intermediate is formed
pair of electrons in C-H bond moves back into ring
step three: regenerate H2SO4 catalyst

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17
Q

halogenation of benzene

A

reagents: halogen (Cl, Br) and a halogen carrier (catalyst) - produces electrophile
conditions: reflux in the presence of halogen carrier
equation: C6H6 + Cl2 –> C6H5Cl + HCl

18
Q

halogenation of benzene mechanism

A

step one: halogen ion produced
step two: ion accepts pair of electrons from benzene ring - forms dative covalent bond
organic intermediate is unstable - breaks down to become stable and relase a H+ ion
step three: hydrogen ion reacts with halogen carrier to regenerate catalyst

19
Q

production of halogen ion: halogenation of benzene

A

halogen + halogen carrier –> halogen ion + halogen carrier ion
e.g. Br2 + FeBr3 –> Br+ + FeBr4 -

20
Q

regeneration of catalyst: halogenation of benzene

A

H+ + halogen carrier ion –> halogen carrier + acid
e.g. H+ + FeBr4 - –> FeBr3 +HBr

21
Q

alkylation of benzene

A

increases no. of C atoms by forming carbon-carbon bonds
reagents: haloalkane, AlCl3 (catalyst) - produces electrophile
substitution of H atom by alkyl group

22
Q

acylation of benzene

A

reacts with acyl chloride
presence of AlCl3 (catalyst)
forms aromatic ketone

23
Q

why is a halogen carrier req. to react bromine with benzene

A

benzene = delocalised pi-electrons spread above and below the plane of ring
low electron density
Br2 = non-polar, insufficient pi-electron density around 2 carbon atoms to polarise Br2

24
Q

explain resistance to bromination of benzene compared with alkenes

A

benzene: electrons delocalised
alkenes: electrons localised
benzene has a lower electron density than alkenes
benzene attracts bromine less
benzene induces weaker dipole in bromine

25
Q

why does bromine/nitric acid react more readily with phenol than benzene

A

increased reactivity: lone pair of e- in p-orbital of O in OH group partially dissociate into ring
increases electron density
electrophile more polarised

26
Q

why is phenol less soluble than other alcohols

A

presence of non-polar benzene ring
phenol partially dissociates when dissolved in water, forming a phenoxide ion and a proton
phenol is classed as a weak acid

27
Q

acidity of phenol

A

more acidic than alcohols, less acidic than carboxylic acids
comparing Kₐ

28
Q

how to distinguish between phenol and carboxylic acid

A

sodium carbonate
carboxylic acid reacts to produce CO2 gas

29
Q

bromination of phenol

A

forms white precipitate - 2,4,6 tribromophenol
reaction decolourises bromine
no halogen carrier required

30
Q

nitration of phenol

A

+ dilute HNO3
2-nitrobenzene and 4-nitrobenzene forms
2-nitrobenzene = major product

31
Q

compare reactivity of phenol and benzene

A

bromine and nitric acid react more readily with phenol
phenol: dilute nitric acid VS benzene: concentrated nitric acid
phenol more reactive due to lone pair of e- on oxygen p-orbital of OH group donated into π-system of phenol = increases electron density of benzene ring = more electrophiles attracted

32
Q

phenol reacts with NaOH

A

forms O-Na+ on aromatic ring
+H2O

33
Q

electron-donating groups

A

donate electron density into pi system of benzene = making it more reactive
2,4 directing groups

34
Q

electron-withdrawing groups

A

remove electron density from pi system of benzene = making it less reactive
3 directing groups

35
Q

2,4 directing groups

A

-OH, -NH2, -CH3, -X

36
Q

3 directing groups

A

-NO2, -CN, -COOH, -CHO

37
Q

aldehyde

A
38
Q

ketone

A
39
Q

oxidation

A
40
Q

reduction

A
41
Q

2,4-DNPH

A
42
Q

Tollen’s

A

chemistry (6 decks)

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