o-chem Flashcards
1
Q
evidence for benzene having a delocalised structure
A
- bond length intermediate between short c=c and long c-c
- enthalpy of hydrogenation less exothermic than expected when compared to enthalpy of hydrogenation for kekule structure
- only reacts with br2 at high temp or in presence of a halogen carrier
2
Q
compare benzene to alkenes with bromine
A
- alkenes react easily with bromine at room temp. benzene does not react without halogen carrier
- in benzene electrons in the pi bond are delocalised, in alkenes they are localised between two carbons
- benzene therefore has a lower electron density, and polarises br2 less and induces a weaker dipole in bromine
3
Q
nitration of benzene
- forms?
- reagents
- mechanism
- electrophile
- equation for formation of the electrophile
- overall equation
- mechanism drawing
A
- nitrobenzene
- conc nitric acid in the presence of conc sulfuric acid
- electrophilic substitution
- NO2+
4
Q
what is the importance of nitrating benzene
A
useful in
TNT
the formation of arenes
5
Q
why is nitration of benzene done at 60 degrees
A
at higher temps, a second nitro group can be substituted onto different positions on the ring
6
Q
halogenation of bromine and chlorine
- forms?
- conditions
- mechanism
- equation for formation of electrophiles
- overall equation
- mechanism drawing
A
- bromobenzene/chlorobenzene
- for bromine = febr3
for chlorine = alcl3 - electrophilic substitution
7
Q
benzene alkylation
- forms?
- reagents
- conditions
- mechanism
- equation for formation of electrophiles
- overall equation
- mechanism drawing
A
- alkylbenzene
- chloroalkane in the presence of anhydrous aluminum chloride catalyst
- heat under reflux
- electrophilic substitution
8
Q
benzene acylation
- forms?
- reagents
- conditions
- mechanism
- equation for formation of electrophiles
- overall equation
- mechanism drawing
A
- phenyl ketone
- acyl chloride in the presence of anhydrous aluminum chloride catalyst
- heat under reflux (50 degrees)
- electrophilic substitution
9
Q
where will substitutions occur when OH or NH2 are present
A
2 and 4 positions on the ring
10
Q
where will substitutions occur when NO2 is present
A
position 3 on the ring
11
Q
benzene molecular formula
A
c6h6
12
Q
benzene
- shape
- bond angle
A
planar
120 degrees
13
Q
how does a sigma bond form in alkanes
A
one sp3 orbital from each carbon overlap
14
Q
alkanes
- bond angle and shape
A
tetrahedral as all four bond pairs repel equally
109.5
15
Q
why does bp increase as alkane size increases
A
number of electrons increases
increase in size of induced dipole dipole interactions
16
Q
why do branched alkanes have lower bp compared to straight chain alkanes
A
larger surface area of contact in straight chains, therefore stronger induced dipole dipole interactions
17
Q
why do alkanes have a low reactivity
A
- high bond enthalpy of the c-c bond and c-h bond
- low polarity of sigma bonds
18
Q
what are the economic reasons for cracking.
A
- fractions with shorter c chains are in more demand
- to make use of excess larger hydrocarbons and to supply demand for shorter ones
- the products of cracking are more valuable than the starting materials
19
Q
conditions for cracking
A
slight pressure
high temp (450 degrees)
zeolite catalyst
20
Q
condition required for synthesis of chloroalkanes
A
uv light
21
Q
how are sigma and pi bonds formed in alkenes
A
sigma - two sp2 orbitals overlap
pi - sideways overlap of two p orbitals
22
Q
alkenes
- angle
- shape
A
120 degrees
planar
23
Q
what are the effects of e-z stereoisomerism on physical properties
A
e - molecule is non-polar, only contains london forces so bp is lower
z - polar, IMF are london and permanent dipole dipole, so bp is higher
24
Q
alkenes with hydrogen reaction
- conditions
- type of reaction
A
- h2 and nickel catalyst
- addition
25
alkenes with bromine/chlorine reaction
- reagent
- type of reagent
- conditions
- mechanism
- type of bond fission
- draw the mechanism
- br2
- electrophile, Br+
- room temp
- electrophilic addition
- heterolytic
26
alkenes with HBr
- reagent
- type of reagent
- conditions
- mechanism
- draw the mechanism
- HBr
- electrophile , H+
- room temp
- electrophilic addition
27
alkenes with steam reaction
- what does it form
- reagent
- conditions
- type of reaction
- alcohols
- steam
- high temp (300-600)
high pressure (70atm)
conc h3po4
- hydration as water is ADDED
28
ways to deal with waste polymers
- seperation and recycling
- feedstock for cracking
- combustion for energy production
29
why do alcohols have low volatility
due to their ability to form h bonds between alcohol molecules
30
why are smaller alcohols soluble in water
due to their ability to form h bonds with the water
31
what are the uses of alcohols
ethanol - solvent in methylated spirits
methanol - petrol additive to improve combustion, feedstock in production of organic chemicals
32
partial oxidation of primary alcs
- forms ?
- reagents
- conditions
- observation
- draw equation for oxidation of propan-1-ol to propanal
- aldehyde
- potassium dichromate and dilute sulfuric acid
- heat and gently distil
- orange dichromate ion reduces to the green Cr3+ ion
33
what colour does k2cr207 go during oxidation
orange turns green
34
full oxidation of primary alcs
- forms?
- reagents
- conditions
- draw equation for oxidation of propan-1-ol to propanoic acid
- carboxylic acid
- potassium dichromate and dilute sulfuric acid
- heat under reflux
35
oxidation of secondary alcs
- forms?
- reagents
- conditionsdraw equation for oxidation of propan-2-ol to propanone
- ketone
- potassium dichromate and dilute sulfuric acid
- reflux
36
reaction of alcs with dehydrating agents
- forms?
- reagents
- type of reaction
- alkene and water
- conc sulfuric acid or phosophoric acid
- heat under reflux
- elimination reaction
37
substitution reaction of alcohols
- forms?
- reagents
- haloalkane
- conc sulfuric and sodium halide
38
nucleophilic substitution of haloalkanes
- forms?
- reagent
- conditions
- mechanism
- draw the mechanism
- alcohol
- potassium hydroxide
- in aq solution, heat under reflux
- nucleophilic substitution
39
why are smaller carbonyls soluble in water
they can form H bonds with water
40
oxidation of aldehydes
- forms?
- reagent
- conditions
- observation
- equation
carboxylic acid
potassium dichromate and dilute sulfuric acid
heat under reflux
orange dichromate ion reduces to the green cr3+ ion
41
tollens reagent
- made of?
- conditions
- reaction
- observation
- equation
- aqueous ammonia and silver nitrate
- heat gently
- aldehydes are oxidised into a carboxylic acid. silver(I) ions are reduced to silver atoms
- silver mirror forms coating the inside of the test tube
42
reduction of carbonyls
- reagents
- conditions
- what do aldehydes form when reduced
- what do ketones form when reduced
- draw mechanism
- type of mechanism
NaBH4 in aq ethanol
room temp and pressure
primary alcohols
secondary alcohols
nucleophilic addition
43
catalytic hydrogenation
- type of reaction
- reagent
- conditions
- reduction
- h2 and nickel catalyst
- high pressure
44
formation of hydroxynitriles
- reagent
- conditions
- mechanism
- why don't we use HCN
- draw mechanism
- NaCN and dilute H2SO4
- room temp and pressure
- nucleophilic addition
- toxic gas that is difficult to contain
45
reaction with 2.4 DNP
- what does it react with
- observation
- how to identify which carbonyl was used
- aldehydes and ketones
- orange precipitate
- take the melting point of the orange crystals, and compare with known values in the database
46
name of the process by which a chlorine molecule splits into two free radicals
homolytic fission
47
free radical definition
a reactive species that possesses an unpaired electron
48
write mechanism for free radical substitution of Br2 and propane
49
why are pi bonds vulnerable to attack in alkenes
- exposed and have high electron density
50
when does e z stereoisomerism arise
- when there is restricted rotation around the double bond
- when there are two different groups attached to both ends of the double bond
51
why are alkenes reactive
due to relatively low bond enthalpy of the pi bond
52
why do alkenes undergo addition reactions
- the double bonds are areas with high electron density - this attracts electrophiles and therefore they undergo addition reactions
53
what is markonwikoffs rule
in most cases, the bromine eg will attach to the carbon with the fewest hydrogens on it
54
what is the order of stability for carbocations and why
tertiary > secondary > primary
- because methyl groups on either side of the positive carbon are electron releasing and reduce the charge on the ion and stabilise it
55
draw the addition polymer of ethene
- explain why poly(alkenes) are unreactive
- due to the strong C-C and C-H bonds
56
what are the industrial importances of alkenes
- formation of polymers from ethene based monomers
- manufacture of margarine by catalytic hydrogenation of unsaturated veg oils using hydrogen and a nickel catalyst
57
alcohols general formula
CnH2n+1OH
58
how to maximise yield during distillation
only collect the distillate at the approximate boiling point and not higher
collection flask can also be cooled in ice
59
why can tertiary alcohols not be oxidised
- no hydrogen atom bonded to the carbon with the OH group
60
equation for the formation of ozone
O + O2 →O3 (reversible)
61
destruction of ozone layer by chlorine free radicals equation
destruction of ozone layer by NO equations
on chemrevise
62
describe benzenes structure
- 6 carbon atoms in a hexagonal ring with one H bonded to each C atom
- each C atom is bonded to two other C atoms and one H atom by single covalent sigma bonds
- this leaves one unused electron on each C atom in a p orbital
- the six p electrons are delocalised in the ring structure above and below the plane of carbon atoms
63
benzene formula
C6H6
64
benzene
- shape
- bond angle
planar
120 degrees
65
draw enthalpy of hydrogenation diagram to represent benzene
chemrevise
66
phenol reaction with bromine
- reagents
- conditions
- draw overall reaction
- observations
- Br2
- room temp
- bromine water decolourises and a white precipitate is formed
67
phenol reaction with nitric acid
- reagent
- conditions
- overall reaction
- 4M HNO3
- room temp
68
solubility of carboxylic acids in water
smaller carboxylic acids (up to C4)dissolve in water but after this solubility decreases. they dissolve as they can form hydrogen bonds with the water molecules
69
test for carboxylic acids
add sodium carbonate and there is effervescence
70
why do aliphatic amines act as a base
why don't aromatic amines act as a base
- lone pair of electrons on the nitrogen is readily available for forming a dative covalent bond with a H+ and so accepts a proton
- don't form basic solutions as the lone pair of electrons on the nitrogen delocalise with the ring of electrons in he benzene ring, therefore N is able to accept less protons
71
amines + acids
- form?
- how to convert back into an amine?
ammonium salts
add NaOH to the ammonium salt to convert back to an amine
72
amine + ammonia
- forms?
- reagent
- draw the mechanism
- primary amine
- excess ammonia is the reagent (this limits further substitution and maximises the amount of primary amine formed
73
how to make secondary and tertiary amines
secondary: primary amine + haloalkane
tertiary: secondary amine + haloalkane
74
reducing nitroarenes
- forms?
- reagent
- conditions
- mechanism
- draw equation
- aromatic amine
- Sn and conc HCL
- heating under reflux
- reduction
75
draw the general structure of an alpha amino acid
- why is it called an alpha amino acid?
- both the NH2 and COOH groups are joined to the same carbon
76
explain why the enthalpy of hydrogenation of benzene is lower than expected
The delocalisation (of electrons) in the benzene ring increases the stability
77
why is methyl benzene more reactive than benzene
- methyl group is electron donating
- Making the benzene ring more able to attack electrophiles
78
amines reaction with acids
- forms?
- how to convert back to amine?
- write equation for reaction of butylamine and HCl
- ammonium salts
- add NaOH
- CH3CH2CH2CH2NH2 + H20 → CH3CH2+NH3 + OH-
79
draw mechanism for the reaction of a primary amine with excess ammonia dissolved in ethanol and a haloalkane
- why is excess ammonia used
- what other conditions are needed
- to limit further substitution maximising the amount of primary amine formed
- hot and under pressure
80
what is a tertiary amine
an organic molecule with three R groups attached
81
amines can be produced from haloalkanes, explain how a mixture of primary, secondary, tertiary and quaternary amines are produced in these reactions
More than one hydrogen on the nitrogen atom from ammonia can be substituted
82
functional group of a primary amide
-CONH2
83
general formula of an alpha amino acid
RCH(NH2)COOH
84
do condensation or addition polymers decompose better? and why?
- condensation polymers
- they can be hydrolysed into their component monomers
85
hydrolysis reaction definition
the breakdown of a compound in the presence of an aqueous solution
86
describe the reaction conditions and state what happens to the polyester during acid hydrolysis
The polyester is heated under reflux in the presence of sulfuric acid
- the polyester breaks down with the acid acting as a catalyst
87
outline the difference in the products between acid and base hydrolysis of a polyester
Acid hydrolysis produces a dicarboxylic acid
Base hydrolysis produces the salt of a dicarboxylic acid
88
why are addition polymers less reactive than condensation polymers
-only contain single bonds
-non polar so chemically inert
- polyesters contain polar bonds so can react with acids and alkalis
89
what is the product when cyclohexene and bromine react together
1,2 dibromocyclohexane
90
Explain the relative resistance to bromination of benzene compared to
phenol and compared to cyclohexene.
- benzene electrons/pi bonds are delocalised
- phenol: the OH group is partially delocalised into the ring
-cyclohexene electrons are localised
- benzene has a lower electron density than phenol or cyclohexane
- benzene cannot polarise or induce a dipole in Br2
91
how does a halogen carrier allow a reaction to take place
induces a permanent dipole
92
Explain the relative resistance to bromination of benzene compared to phenol and compared to cyclohexene
greater reactivity of phenol....
(the ring is activated because …)
lone pair from O is delocalised into the ring (1)
so electron density (of the ring) is increased (1)
so electrophiles are more attracted (to the ring)
93
delocalised π-bond electrons definition
- electrons are spread over more than two atoms
- pi bond formed by overlap of p orbitals
94
why do aldehydes give a different reaction to tollens reagent than ketones
aldehydes can be oxidised to a carboxylic acid
95
how does gas chromotography separate substances
relative solubility in stationary phase
96
amino acid general formula
RCH(NH2)COOH
97
Name the process by which TLC separates α-amino acids
adsorption
98
rf value definition
Rf value is distance moved by a component/spot/solute divided
by distance moved by solvent
99
retention time definition
Retention time is the time between injection and emergence
(or detection) of a component
100
what are the roles of the gas and liquid in chromatography
gas: mobile phase
liquid: stationary phase
101
State the physical process on which the separation used in gas-liquid chromatography depends
partition
102
recrystallisation method
- dissolve the impure compound in a minimum amount of hot solvent
- hot filter solution through filter paper
- cool the filtered solution by inserting beaker in ice
- suction filtrate with buchner flask to separate out crystals
- wash crystals with distilled water
- dry between absorbant paper
103
State and explain the effect of the –OH group on the reactivity of the benzene
ring in phenol.
- lone pair from o atom is delocalised into the ring
- greater electron density around the ring
- attracts electrophiles
104
why do optical isomers form
- if there is a chiral carbon
- mirror images cannot be superimposed
105
what do optical isomers have in common and what varies?
similar physical and chemical properties
rotate plane polarised light in different directions
106
what is the isoelectric point
the ph at which the overall charge of a molecule is zero
107
why does the C=O double bond aid biodegradability
absorbs radiation
ester linkage hydrolysed
108
Explain how the student could analyse the chromatogram to identify the three molecules present
- measure how far each spot travels relative to the solvent front/calculate the rf value
- compare rf values to those known for the molecules
109
Explain how the gas/liquid chromatogram could be used to determine the
percentage composition of each component in the mixture.
measure area under each peak
find total area
110
State what quantitative value is normally used to identify the components in gas liquid chromatography AND State the physical process on which the separation used in gas-liquid
chromatography depends.
retention time
partition
111
how to purify an organic liquid
- put the distillate of impure product into a separating funnel
- wash by adding saturated sodium chloride solution
- allow the layers to separate in the funnel, and the run and discard the aqueous layer
- run organic layer into a clean dry conical flask and add drying agent
-decant liquid into distillation flask
- distill to collect pure product
112
molecular ion peak meaning
base peak meaning
- the peak caused by the unfragmented molecule
- peak with the greatest relative intensity
113
what happens when D2O is added to a sample
- can be used to identify N-H and O-H peaks
- protons bonded to electronegative atoms will be able to hydrogen bond to D2O
- therefore peak will disappear
114
esterification
- two molecules needed?
- produces?
- reagents
alcohol + c acid
ester and water
strong sulfuric acid catalyst
115
hydrolysis of esters
1. with acid
- HCl dilute and heat under reflux
- produces c acid and alcohol
2. with dilute NaOH
- heat under reflux
-produces salt and alcohol
116
rate of substitution in the halogens
- iodoalkanes are fastest to substitute
- fluoroalkanes are slowest
- the strength of the C-F bond means that they are very unreactive
117
how to compare the rate of hydrolysis reactions in haloalkanes
- aq silver nitrate added
- forms a silver halide precipitate
- rate of the formation of the precipitate can be used to compare reactivity of the different haloalkanes
118
reactivity of benzoic acid
COOH is an electron withdrawing group
electron density is lower
therefore less susceptible to attack
