organic chemistry - AS Flashcards
1
Q
define homologous series
A
family of compounds with similar chemical properties, whose successive members differ by addition of a CH2 group
2
Q
define functional group
A
part of organic molecule thats responsible for the molecule’s chemical properties
3
Q
define aliphatic
A
it is chain
- when carbon atoms are joined together in branched or unbranched chains or non aromatic rings
4
Q
define alycyclic
A
when carbon atoms are joined in cyclic structures
5
Q
define aromatic
A
when theres a benzene ring
6
Q
homolytic fission
A
breaking covalent bond so that each atom takes an electron from the bond to form two radicals
7
Q
define radical
A
highly reactive chemical species with an unpaired electron
8
Q
define heterolytic fission
A
breaking covalent bond so the more electronegative atom takes both electrons from the bond to form a negative ion and leave behind a positive ion.
9
Q
what are some of the problems with free radical substitution
A
- get a whole range of side products (low atom economy?) -> this is because rhe haloalkane formed in propogation step 2 can go on to react further the radical and so on, so at end of the reaction we need to separate out the product molecules
- in the termination step, other impurities can also be formed
- also, theres a problem with the halide geting substituted at different points on the carbon chain, with longer chains.
10
Q
shape and bond angel of alkane molecule
A
tetrahedral, 109.5 degrees
11
Q
empirical formula
A
simplest whole number ratio of atoms of each element in a compound
12
Q
molecular formula
A
- actual number of atoms of each element in a molecule
13
Q
displayed formula
A
shows how all atoms are arranged and all the bonds between them
14
Q
aldehyde suffix
A
-al
- eg: butanal
15
Q
what is a structural isomer
A
- compounds with same molecular formula bur different structural formula
- three types of structural isomers:
- chain, positional and functional group isomers
16
Q
chain isomers
A
- when carbon skeleton chain can be arranged differently, for example as a straight chain or branched
- have similar chemical properties but their physical properties like boiling point will be different because of the change in shape of molecule
17
Q
positional isomers
A
- skeleton and functional group could be the same, only with functional group attached to a different carbon atom
- also have different physical properties and chemical properties may differ as well
18
Q
functional group isomers
A
- same atoms arranged into different functional groups
- have very different physical and chemical properties
19
Q
shape of alkane molecule
A
- tetrahedral around each carbon
- 109.5 degrees
20
Q
how does boiling point of alkane vary
A
- increases as chain increases
- they have london forces between alkane molecules
- longer chain = stronger london forces (stronger induced dipole-dipole interactions)
- this because more surface contact so more electrons interact
- as molecule gets longer, more energy required to overcome london forces so bp rises
- branched chain has lower bp than its straight chained isomer as they cannot pack closely together so smaller molecular surface area, less interactions, so less london forces
21
Q
why is carbon monoxide bad
A
- oxygen in bloodstream carried by haemoglobin
- CO binds better to haeomoglobin than O
- less O can be carried around body so oxygen deprivation
22
Q
free radical substitution
A
initiation - uv light causes the halogen bond to split equally with each atom keeping one electron (homolytic fission)
- forms a free radical
propogation - free radicals are used up and created in a chain reaction
- there are two steps of this stage
termination - free radicals are mopped up
- two free radicals join together and make a stable molecule
23
Q
problems with free radical substitution
A
- u get a mixture of products, not just the product that you want
- the propogation reaction causes this, with lots of diff substituton reactions taking place
- the end product with the mixture is a disadvantage as u need to separate the acc product from the numerous by products
- free radical substitution can also take place on any point of the carbon chain, so a mixture of isomers are produced
24
Q
describe the nature of the double bond
A
- made of sigma and pi bonds
- sigma bond formed when two s orbitals overlap.
- they overlap in a straight line, so this has highest possible electron density.
- the high electron density between nuclei means theres strong electrostatic attraction between the nuclei and the shared pair of electrons, so sigma bonds have high bond enthalpy, strongest covalent bond
- pi bond is formed from the sideways overlap of two adjacent p orbitals
- its got two parts to it - one above and one below the molecular axis because theyre dumbbell shaped.
- pi bonds weaker than sigma bonds as electron density is spread out above and below nuclei, so electrostatic attraction between nuclei and shared pair of electrons is weaker, so lower bond enthalpy
25
why are alkenes more reactive than alkanes
- alkanes only have sigma bonds, which have high bond enthalpy so difficult to break
- theyre also non polar so dont attract nucleophiles or electrophiles, so they do not react easily
- alkenes more reactive as the c=c bond contains both pi and sigma bonds
- the c=c bond also contains four electrons, so high electron density, and the pi bond sticks out above and below the rest of the molecule, so more likely to be attacked by electrophiles
- pi bonds have low bond enthalpy as well
26
why can double bonds not rotate
- because of the way that the p orbitals overlap to form pi bond
- this restricted rotation around the c=c double bond is what causes alkenes to form sterioisomers
27
what are stereoisomers
- compounds w same structural formula but diff spacial arrangement
28
e isomers
- entgegen: opposite
- so they have same groups positioned across double bond, on diff sides
29
z isomers
- zuzammen: together
- have the same grps positioned on thte same side (either below or above the double bond)
30
which rule can be used to work out the e and z isomer of any alkene
cahl-ingold-prelog rule
31
cis isomer
- same groups are on same side of double bond
32
diff between e/z isomers and cis/trans isomers
- cis/trans can only be used when the carbon atoms have the same groups attached
- but e/z can be used even when carbon atoms have all diff groups attached to them
33
what are electrophiles
- electron pair acceptors
34
alkene -> alkane conditions and reaction
- hydrogen gas
- nickel catalyst at 150 degrees celsius
35
alkene -> alcohol
- steam
- 300 degrees celsius, 60-70 atm pressure
- H3PO4 (phosphoric acid catalyst)
36
alkene -> dihaloalkane
- electrophilic addition
- halogens end up across the double bond
STEPS:
1. the halogen is non polar, but interactions between the pi bond electrons in double bond and the bromine causes polarisation - **induced dipole**
2. double bond breaks as the electron pair is attracted to the slightly positive halide
3. the halogen bond breaks by **heterolytic fission** and the electron pair goes towards the negative halide
37
alkene -> haloalkane
- with hydrogen halide
- electrophillic additon, this time the hydrogen is less electronegative than the halide, and so polar, and so attracted to the pi bond
38
why does adding hydrogen halides to unsymetrical alkenes form more than one product
- it forms different carbocations during the reaction
- carbocations with more alkyl groups are more stable because alkyl groups feed electrons towards the positive charge, and more stable carbocation is more likely to form
- secondary -> the positive charge is on the carbon that has 2 other carbons attached/two other alkyl groups attached
39
what is markownikoff's rule
- major product from addition of a hydrogen halide to an unsymmetrical alkene is the one where hydrogen adds to the carbon with most hydrogens already attached
40
why do alcohols generally have higher boiling points than their corresponding alkanes
- have weak london forces between their non polar groups
- but also have OH groups which are polar due to the difference in electronegativity between O and H
- means they have hydrogen bonds as well, which are much stronger
41
why are alcohols soluble in water
- have OH group which is polar
- they form hydrogen bonds between the water molecules
- as hydrocarbon chain of the alcohol increases, solubility decreases as the influence of OH group decreases
42
alcohol -> haloalkane
- react with any compound containing halide ions
- substitution reaction
- need an acid catalyst (normally concentrated sulfuric acid at room temp)
43
alcohol -> alkene
- dehydration reaction
- concentrated acid catalyst like conc sulfuric acid or conc phosphoric acid, then heat
- forms alkene and water
- done under reflux
44
test for double bonds
- bromine water
- orange to colourless
- this happens because the bromine is added across the double bond to form dibromoalkane
45
oxidation of primary alcohol
- under distillation forms aldehyde
- under reflux forms carboxylic acid
- oxidising agent: acidified potassium dichromate
46
oxidation of secondary alcohol
- under reflux to form ketone
47
oxidation of tertiary alcohol
- they do not react with potassium dichromate at all so solution stays orange (doesnt change to green)
- only way to oxidise is by burning
48
what is a nucleophile
electron pair donor
49
haloalkane -> alcohol
- nucleophilic substitution
- warm aqueous sodium hydroxide or potassium hydroxide
- under reflux
- haloalkanes generally insoluble in water so ethanol is added to reaction
STEPS:
1. OH- is the nucleophile tht provides the pair of electrons for the slightly positive carbon that is bonded to the halide *(this because the carbon-halogen bond is polar, halogens more electronegative than carbon)*
2. C-Halide bond breaks **heterolitically** (both electrons taken by the Halide ion)
3. the halide ion falls off and the hydroxide ion bonds to the carbon
50
examples of nucleophiles
- water
- hydroxide ion (OH-)
- ammonia
51
how does rate of hydrolysis change with the haloalkanes
- iodoalkanes are hydrolysed fastest
- the bond enthalpy of the carbon-halogen bond decreases down the group
- weaker carbon-halogen bonds break faster, so they react faster
52
experiment to measure rate of hydrolysis
1. place one cm cubed of ethanol into test tube
2. add 0.1 cm cubed of haloalkane to each test tube
3. place in waterbath of 60 degrees c
4. add aqueous silver nitrate to diff test tube and place in same waterbath + wait for them all to acclimatise
5. add 1cm3 of the silver nitrate to each test tube and start stopwatch
6. the halide ions and silver ions react immediately as they form to form silver halide precipitate
7. observe: pale yellow forms quickest -> iodoalkanes are most reactive
- nucleophilic substitution happens here as its aqueous silver nitrate, so the solvent which is water reacts.
53
CFC properties, uses and problems
- stable, volatile, non-flammable, non-toxic
- used in fridges, aerosol cans, dry cleaning and air conditioning
- they destroy ozone layer.
54
steps of how CFCs cause ozone depletion
1. C-Cl bond breaks to form Cl● due to UV light from sun
2. the free radicals are catalysts. react w ozone (O3) and form oxygen and an intermediate
- Cl● + O3 -> O2 + ClO●
- ClO● + O -> O2 + Cl●
OVERALL:
O3 + O -> 2O2
*Cl● acts as a catalyst for the reaction*
55
how nitrogen oxides also break down ozone
- NO● formed from nitrogen oxide
- react in same way as chloride radicals
- theyre produced from thunderstorms, aircraft engines and car
