orgo Flashcards
why do alkanes have a low reactivity?
- strength of C-C and C-H bonds
- non polar so not susceptible to attack
heterolytic fission
results in the formation of a +ve and -ve ion
homolytic fission
results in the formation of two radicals
define a radical
reactive species which possess an unpaired electron
define free radical substitution
a type of substitution where a radical replaces an atom/group of atoms
why do alkanes undergo free radical substitution
they have a low reactivity
why do we use UV light for free radical substitution?
it is strong enough to break the Cl-Cl or Br-Br bonds but not C-C or C-H bonds
describe the mechanism for free radical substitution
limitations of free radical substitution
not effective for creating a specific halogenoalkane
- any of the hydrogens can be substituted
- multi substitutions can also occur
what type of reaction do alkenes undergo
electrophilic addition
why do alkenes undergo electrophilic addition
- double bond has a high electron density so attracts electrophiles
- pi bond can break to form 2 new bonds
define an electrophile
an electron deficient species that can accept an electron pair from a nucleophile
describe the bromine water test for alkanes and alkenes
alkanes remain orange
alkenes go from orange to colourless
give the mechanism for electrophilic addition for alkenes
alkene + hydrogen
nickel catalyst, 180’C
alkane
alkene + steam
phosphoric V acid
300’C
alcohol
alkene + halogen
in hydrocarbon solvent
vicinal dihalide
describe Markvnokov’s rule
in an asymmetric alkane undergoing an electrophilic addition reaction with a hydrogen halide, the H atom will add to the C atom that is bonded to the most hydrogen atoms
- in an intermediate carbocation, the positive inductive effect of the R groups pushes electrons towards the positive carbocation, stabilising it
when do alcohols combust and what does this produce
in plentiful supply of oxygen to form carbon dioxide and water
dehydration of alcohols
alcohol -> alkene + water
phosphoric acid, heat under reflux and collect the product (alkene) by distillation due to its lower boiling point
partial oxidation of primary alcohol
primary alcohol + oxygen -> aldehyde + water
- K2Cr2O7 (potassium dichromate)
- sulphuric acid
- heat and distil off the product as soon as it is formed to prevent further oxidation
complete oxidation of primary alcohols
primary alcohol + oxygen -> carboxylic acid + water
- excess K2Cr2O7 (potassium dichromate)
- sulphuric acid
- heat under reflux for at least 10 minutes then distil off the product
oxidation of secondary alcohols
secondary alcohol + oxygen -> ketone + water
heat with potassium dichromate, dilute sulphuric acid
potassium dichromate colour change
orange to green (oxidised)
acidified potassium manganate (KMnO4) colour change
purple to colourless
esterification
alcohol + carboxylic acid -> ester + water
- heat in water bath with concentrated sulphuric acid
carbonyl group vs carboxyl
C=O vs COOH
define a chiral carbon
carbon attached to 4 different atoms/groups, (optically active)
optical isomers
enantiomers, or non-superimposable mirror images of each other
optically active compound
a compound that can rotate the plane of plane-polarised light and be distinguished using a polarimeterr
racemic mixture
contains equal amounts of both enantiomers - optically inactive as the two rotations in opposite directions cancel each other out
physical and chemical properties of enantiomers
physical : identical except the ability to rotate the plane of plane polarised light in opposite directions
chemical : identical except when they interact with other optically active substances
diastereomers
not mirror images of each other. contain different configurations at one or more, but not all, of the equivalent stereocenters
define a nucleophile
an electron rich species which can donate a lone pair of electrons to an electrophile
what type of mechanism do primary halogenoalkanes undergo?
Sn2
draw out the mechanism for Sn2
in Sn2, what is involved in determining the ror?
both the halgenoalkane and the nucleophile
why do primary halogenoalkanes react via Sn2?
the small space occupied by the two hydrogen atoms allows room for the nucleophile to approach the central carbon atom
what mechanism do tertiary halogenoalkanes undergo?
Sn1
draw out the mechanism for Sn1
why do tertiary halogenoalkanes undergo Sn1?
the positive inductive effect of the R groups pushes electrons towards the central carbon ion, stabilising it, allowing it to form
what determines the rate of Sn1
the halogenoalkane
factors affecting the rate of nucleophilic substitution
type of halogenoalkane
nature of halogenoalkane
type of solvent
type of halogenoalkane
primary < secondary < tertiary
tertiary quickest as they undergo an ionic mechanism, so Sn1 faster than Sn2
nature of halogenoalkane
- c-cl is stronger than c-br
- c-br is a better leaving group as the bond takes less energy to break
type of solvent
Sn1: protic, polar solvents (eg water, ethanol)
- contain N/O atoms bound to H
- solvate both the carbocation and the halide ion well, stabilising them and making them more easily formed
Sn2: non-protic, polar solvents (eg propanone)
- do not contain N/O atoms
- does not solvate nucleophile well, leaving it free to attack the halogenoalkane
describe benzene
- 6 carbon atoms, 6 hydrogen atoms
- hexagonal planar molecule
- each c atom is sp2 hybridised and forms a sigma bond w each of the 2 neighbouring c atoms and a sigma bond with a hydrogen atom
- 6p orbitals contain an electron each and form the delocalised pi bond above and below the plane of the molecule
physical evidence for the structure of benzene
- bond lengths: if benzene contained both C-C and C=C bonds, it would be expected that the bonds be different lengths, some shorter than others. However, all the bonds are the same length, which is inbetween the length of a C-C and a C=C bond
- Enthalpy of hydrogenation: enthalpy change of hydrogenation/combustion is less exothermic than
predicted for cyclohexa-1,3,5-triene, which accounts for the extra energy needed to overcome the resonance, or delocalisation energy of the molecule
chemical evidence for the structure of benzene
benzene undergoes substitution reactions much more readily that addition reactions. If it contained c=c double bonds, it would be expected to undergo addition reactions readily. this is because the delocalisation, or resonance energy, needs to be overcome before molecules can add
draw the mechanism for the nitration of benzene
conditions needed for nitration of benzene
conc sulphuric acid to protonate the HNO3 to form the electrophile, No2+
50’c (at temperatures higher than this, 1,2 dinitrobenzene is formed)
describe lithium aluminium hydride
LiAlH4
- stronger reducing agent (can be used for carboxylic acids)
- must initially be used in aprotic solvent (eg ether) as it reacts vigorously with water to release hydrogen
- once the intermediate product has formed, it can be reacted with water to form the product
describe sodium borohydride
NaBH4
- can be used in protic solvents but is not strong enough to reduce carboxylic acids
reduction of aldehyde
aldehyde -> primary alcohol
either, H+ (aq)
reduction of ketone
ketone -> secondary alcohol
either, H+ (aq)
reduction of carboxylic acid
carboxylic acid -> primary alcohol
LiAlH4 in ether
H+ (aq)
describe the reduction of nitrobenzene to phenyl amine
- reduction of tin to produce C6H5NH3+
- heat under reflux w tin, and conc HCl - release of phenyl amine via an acid-base reaction (adding NaOH)
define structural isomers
when the atoms are bonded in a different way
define stereoisomers
have different arrangement of atoms in space, but do not differ in connectivity
give two examples of stereoisomers
conformational - convert by rotation around a sigma bond
configurational - convert by breaking and reforming a bond (optical, E/Z)
