Organics Flashcards
1
Q
What is a hydrocarbon?
A
A compound containing only carbon and hydrogen
2
Q
What is the empirical formula?
A
The simplest whole number ratio of the elements present in a compound.
3
Q
What is the molecular formula?
A
The total number of atoms of each element present in a molecule of the compound. A molecular formula is an integer multiple of the empirical formula.
4
Q
What are the names of the alkanes in increasing carbon atoms?
A
Methane Ethane Propane Butane Propane Butane Pentane Hexane
5
Q
What is a homologous series?
A
A homologous series is a series of compounds that have the same functional group. Each member differs from the next by a common structural unit.
6
Q
What is a functional group?
A
A functional group is the atom or group of atoms in a molecule that gives it its characteristic chemical properties - this is the reactive part of the molecules.
7
Q
What is the formula of alkanes?
A
C n H 2n+2
8
Q
What is the formula of alkene?
A
C n H 2n
9
Q
What is the alkene functional group?
A
C=C
10
Q
What is the alkynes functional group?
A
C-=C (triple bond)
11
Q
What is the alcohol functional group?
A
OH
12
Q
What is the ether functional group?
A
C-O-C
13
Q
What is the aldehyde functional group?
A
O
C
- - H
14
Q
What is the ketone functional group?
A
O
C
- C - C
15
Q
What is the carboxylic acid functional group?
A
COOH
16
Q
What is the amine functional group?
A
NH2
17
Q
What is the ester functional group?
A
C-O-C=O
-
18
Q
What is the nitrile functional group?
A
C-=N (triple bond)
19
Q
What is the amide functional group?
A
O=C-NH2.
20
Q
What are compounds with a benzene ring called?
A
Aromatic
21
Q
What are compounds without a benzene ring called?
A
Aliphatic
22
Q
What does aromatic mean?
A
With a benzene ring
23
Q
What does aliphatic mean?
A
Compounds without a benzene ring.
24
Q
What happens to the boiling point of carbon compounds as carbon length increases?
A
As the number of carbon atoms in a molecule in any homologous series increases, the boiling point increases. The boiling point of straight-chain alkanes increases when a methylene -CH2 group is added, because the strength of the London forces increases as the number of electrons in the molecule does.
25
Which has higher boiling points alkanes or alcohols?
Alcohols because of hydrogen bonding between molecules that is not present in alkanes.
26
Which has a higher boiling point alcohols or esters?
Alcohols because they can form hydrogen bonds. Esters have no hydrogens bonded to oxygen and therefore can not make hydrogen bonds.
27
How does solubility increase or decrease with alcohol's carbon length?
The longer the chain the less soluble because the only soluble bit is the OH on the end and as the chain gets longer that gets less in proportion.
28
How do you name alkanes?
1. Find the longest continuous chain in the molecule, this determines the prefix to the homologous end name (e.g. ane)
2. Look for substituent groups (alkyl groups). Then label the carbons they are on to give the lowest individual numbers (e.g. if it is butane, say it is on carbon 2 instead of 3 because that is lower).
4. Choose a prefix to label how many there are of these substituent groups.
29
How do you name alkenes?
Same as alkanes but the number in the middle
| pen -2- ene indicates the position of the double bond.
30
How do you name halogenoalkanes?
The same as alkanes but put the position of the halogen, then the halogen name and then the alkane.
31
How do you name alcohols?
Same as alkanes except like alkenes but the position of the OH in the middle of the word:
ethan-2-ol.
32
How do you name ethers?
The shorter molecule and the oxygen are called an oxy group, and so if it was 1 carbon and an oxygen it would be methoxy and then the longer one so for example
1- methoxypropane. The 1 is saying that the oxygen and smaller group are on the first of the carbons of the big one.
33
How do you name ketones?
propan-2-one, saying the C=O (with 2 more carbons either side of the carbon) which is the functional group is on the second carbon.
34
How do you name aldehydes?
-al
propanal
pentan-2-al to show that the HC=O is on the second carbon.
35
How do you name carboxylic acids?
Ethanoic acid
36
How do you form an ester?
Esters are formed by a reaction between a carboxylic acid and an alcohol. COOC.
The acid is the second bit of the name, the alcohol is the first.
37
What are the uses of esters?
- Artificial flavours and odours.
| - Plasticisers
38
What is the difference between primary secondary and tertiary alcohols?
They all have different numbers of carbons connected to the carbon which holds the OH group.
If the carbon that holds the OH group is at the end of the chain molecule and is only attached to one carbon then it is primary.
If it is in the middle and that carbon is attached to two others then it is secondary. And if it is attached to three then it is tertiary.
39
What is the difference between primary secondary and tertiary amines?
```
This is slightly different to alcohols, it depends how many alkyl groups are attached to the N, so a primary amine has C-NH2
a secondary amine has
C-NH-C
A tertiary amine has
C-NC-C
```
40
What is a structural isomer?
Structural isomers are two or more compounds that have the same molecular formula but different structural formulas - the atoms are joined together in different ways.
So for example
butane and 2-methylpropane
in 2-methyl propane the end CH3 has just gone above the second carbon. They have the same number of atoms just different ways of putting them together.
41
How do isomers compare boiling point wise?
In general, branched chain isomers have lower boiling points than straight chain isomers. This is because the branches prevent the main chains from getting as close together and so the London forces between the branched molecules are weaker.
42
What are functional group isomers?
Isomers that have the same formula but different functional groups.
for example ethanol and methoxymethane (ether)
43
What is the evidence for the structure of benzene?
The structure has alternating single and double bonds between the carbons. Each carbon is connected to a hydrogen and the rest of the electrons form a delocalised system of six electrons. The ring of electrons is formed when p orbitals overlap side on and form a pi bond.
1. Evidence from carbon carbon bond lengths, all C-C bond lengths are equal in benzene and also intermediate in length between a C-C and a C=C bond. If the structure of benzene was double bond single bond then the bond length would vary significantly.
2. Adding hydrogen to benezene is not three times the enthalpy change of adding hydrogen to C6H10 which is what it would be expected to be if it had three double bonds, it is instead half way between the two.
3. Comes from examining the isomers of C6H4Cl2, there are only 3 isomers we can find, and if there was a double bond single bond then there would be four.
4. Also if it was double bond single bond then it would be expected to undergo addition reactions and to colourise bromine, however it does not. Benzene undergoes substitution instead.
44
What are the disadvantages of benzene?
Benzene is recognised as a human carcinogen which can cause leukaemia. The main source of exposure to benzene in Europe is car exhaust fumes.
45
What is the structure of benzene?
The structure has alternating single and double bonds between the carbons. Each carbon is connected to a hydrogen and the rest of the electrons form a delocalised system of six electrons. The ring of electrons is formed when p orbitals overlap side on and form a pi bond.
46
What are the boiling points of alkanes like?
Alkanes only have london forces and therefore they are volatile and evaporate easily, they are also non-polar so do not dissolve in water.
47
Are alkane soluble?
they are also non-polar so do not dissolve in water.
48
How do alkanes react with oxygen?
Alkanes react with oxygen to produce water and carbon dioxide. They are used as the basis of fuels such as petrol.
When there is a lack of oxygen they undergo incomplete combustion and produce carbon monoxide and water.
49
What are alkanes used for?
Alkanes react with oxygen to produce water and carbon dioxide. They are used as the basis of fuels such as petrol.
50
How reactive are alkanes?
Not very reactive because the C-C bonds are very strong, the C-H bonds are very strong and they are also non-polar and so are unlikely to attract polar ions or molecules.
51
How do alkanes react with halogens?
Alkanes react with halogens in the presence of sunlight or ultraviolet (UV) light. They form a halogenoalkane by free radical substitution.
52
How does free radical substitution work?
Initiation:
The first stage involves breaking apart chlorine molecules into separate chlorine atoms.
Cl2 - 2Cl*
The energy needed to do this is provided by the UV light. The Cl2 bond is weaker than any C-H bond and so it is the Cl-Cl bond that is broken.
These chlorine atoms have only 7 electrons in their outer shell and so each has an unpaired electron, these are called radicals.
This process is called homolytic fission. It is shown using arrows going from one Cl in the Cl-Cl bond to the line.
Propagation:
A chlorine free radical is a very reactive species - when it collides with a methane molecule in the reaction mixture, it will combine with a hydrogen atom to pair up its unpaired electron:
Cl* +CH4 = HCl + *CH3
This creates a methyl radical which will then react with the Cl2 molecule to form a C-Cl bond. The Cl* can react with something else again forming a chain reaction.
Termination:
In the reaction mixture free radicals are always present in very low concentrations so the chance of two colliding is very low. However, they do collide sometimes and this brings the chain reaction to an end. There are several possible termination reactions. Each one involves a decrease in the number of free radicals.
53
What are free radicals?
Free radicals are species (atoms or groups of atoms) with an unpaired electron. Free radicals are very reactive because of this unpaired electron.
54
Which are more reactive and why? Alkanes or alkenes?
Alkenes are more reactive this is because the double bond is not double the strength of the single one because the second component the pi bond is weaker than a normal C-C bond and therefore is more easily broken.
The double bond represents a region of high electron density and therefore it attracts electrophiles.
55
What is an electrophile?
An electrophile is a reagent (a positively charged ion or the positive end of a dipole) that is attracted to regions of high electron density and accepts a pair of electrons (Lewis acid) to form a covalent bond.
56
What is an addition reaction?
It is when the double bond of an alkene breaks and two molecules attach.
57
What happens when a halogen and alkene react?
When an alkene reacts with a halogen it undergoes an addition reaction and turns into a halogenoalkane.
58
What happens when an alkene reacts with hydrogen?
Alkenes react with hydrogen in the presence of a nickel catalyst at 150 degrees to make an alkane, it is called hydrogenation.
59
How does an alkene react with hydrogen halides?
Alkenes react with hydrogen halides such as hydrogen bromide to make a halogenoalkane.
60
How does an alkene react with water?
It forms an alcohol, with heat and concentrated H2SO4 as the catalyst.
61
What is catalytic hydration?
The reaction between an alkene and water and is used in the industrial production of ethanol. It reacts ethene with steam at 300 degrees and 6000kp with a H3PO4 catalyst.
62
What are the melting points of saturated and unsaturated fats like?
Unsaturated fats are liquids at room temperature and called oils. Saturated fats are solids at room temperature and are called fats.
63
How is margarine made?
By hydrogenating, reacting an alkene with water under heat and using H2SO4, this makes the unsaturated fat, saturated and increases its melting point so it is solid, but it only partially hydrogenates it, and where the C=C are not hydrogenised, it produces trans fats which are bad for health.
64
How do you distinguish between alkanes and alkenes?
Alkenes turn bromine water from orange to colourless and alkanes have no colour change and it remains orange.
65
How does benzene react?
Although benzene can be displayed as having a C=C bond, it does not react like an alkene and does not undergo addition reactions but rather it undergoes substitution reactions instead.
66
How does an alkene react with an alkene?
The carbon double bond opens up from a single bond and forms a polymer by polymerisation.
67
What is the economic importance of polymers?
They are used as plastics for everyday life. Making plastic bags, washing up bowls. Polychloroethene is used for making insulation for electrical cables and window frames; and polypropene is used for making car bumpers and carpet fibres.
68
What are the reactions of the alcohols?
PRIMARY
using heat and Cr2O7 2-/H+
Primary alcohols are first oxidised to form an aldehyde (C=O -H)
Then they are oxidised again to form a carboxylic acid (COOH)
SECONDARY
Secondary alcohols are oxidised once by heating with Cr2O72-/H+ to ketones. They only oxidise once.
TERTITARY
Do not react
69
What is the reaction between lots of oxygen and alcohol?
They combust to produce CO2 and H2O.
70
What are the reactions of primary alcohols?
using heat and Cr2O7 2-/H+
Primary alcohols are first oxidised to form an aldehyde (C=O -H)
Then they are oxidised again to form a carboxylic acid (COOH)
What happens to the Cr2O7 2-/H+ when it oxidises the alcohol?
It is reduced, and Cr3+ and H2O are formed.
It changes from orange to green.
71
What is Cr2O7 2-/H+ used for?
To oxidise primary alcohols.
It is reduced, and Cr3+ and H2O are formed.
It changes from orange to green.
72
How do you get from primary alcohol to aldehyde?
You oxidise it, with oxygen and the catalyst Cr2O7 2-/H+, then it turns into an aldehyde but will soon turn into a carboxylic acid so you have to distill it and set up apparatus so that it will be distilled off before it can be oxidised further. This technique works because aldehydes have lower boiling points than the equivalent alcohols (and all other components of a reaction mixture), because there is no hydrogen bonding because there is no hydrogen attached to an oxygen.
73
What are the reactions of secondary alcohols?
Secondary alcohols are oxidised once by heating with Cr2O72-/H+ to ketones. They only oxidise once.
74
What are the reactions of tertiary alcohols?
DO NOT REACT
75
How do you make an ester?
Heat an alcohol with a carboxylic acid under heat and with H2SO4. The H of the alcohol and the OH of the carboxylic acid are lost as water.
This can be described as a nucleophilic substitution reaction where the alcohol acts as the nucleophile and substitutes the OH group of the carboxylic acid.
76
Halogenoalkanes undergo what type of reaction?
Halogenoalkanes undergo nucleophilic substitution.
With water to make alcohols
With NH3 to make an amine.
Primary halogenoalkanes undergo SN2 and secondary halogenoalkanes undergo SN1.
77
What is a nucleophile?
A nucleophile is a molecule or negatively charged ion that has a lone pair of electrons - it is attracted to a relatively highly positively charged region in a molecule (a region with lower electron density) and donates a lone pair of electrons to form a covalent bond.
78
How do primary halogenoalkanes react with water?
They react via the SN2 nucleophilic substitution reaction.
SN2
In a halogenoalkane the halogen will be more reactive than the carbon and therefore the carbon will be slightly positive. The OH- ion will be attracted to it. The hydroxide ion is a nucleophile and will donate its pair of electrons to the carbon atom. The C-Br bond breaks such that both electrons from the C-Br bond go back to the bromine atom to form a bromine ion. This is heterolytic fission (can be contrasted with homolytic fission of Cl in free radical substitution when the electrons go equally whereas here both electrons go to the Br).
Part way through this nucleophilic substitution process both the OH and the Br are joined to the carbon. This is the highest point on the energy profile, this is the transition state.
Then the Br releases. This is a single step mechanism and the formation of the transition state is just part of the way along this step.
USE CURLY ARROWS TO DEMONSTRATE THIS BOTH FROM BOND TO HALOGEN AND OH- TO CARBON.
79
How does an SN2 reaction work?
SN2
In a halogenoalkane the halogen will be more reactive than the carbon and therefore the carbon will be slightly positive. The OH- ion will be attracted to it. The hydroxide ion is a nucleophile and will donate its pair of electrons to the carbon atom. The C-Br bond breaks such that both electrons from the C-Br bond go back to the bromine atom to form a bromine ion. This is heterolytic fission (can be contrasted with homolytic fission of Cl in free radical substitution when the electrons go equally whereas here both electrons go to the Br).
Part way through this nucleophilic substitution process both the OH and the Br are joined to the carbon. This is the highest point on the energy profile, this is the transition state.
Then the Br releases. This is a single step mechanism and the formation of the transition state is just part of the way along this step.
80
How many steps in SN2?
1 - but the reason it is called SN2 is because the rate determining step involves 2 molecules. The rate determining step is the slowest but here there is only one step so it must be the rate determining step.
81
What is the rate equation for SN2?
Only one step, must therefore be rate determining, so it is
R = k[concentration of halogenoalkane][concentration of nucleophile]
This means if the concentration of either is doubled the rate is doubled.
82
How do tertiary halogenoalkanes react?
They react by SN1.
1. In the first step C-Br bond breaks to form a positively charged carbocation. This is the rate determining step.
2. Once formed the carbocation is open to attack by nucleophiles such as the OH- ion.
83
How many steps in SN1?
2.
84
What is the rate equation for SN1?
R = k[concentration of halogenoalkane only]
85
How does the halogen effect the rate of nucleophilic substitution?
Iodine will be the fastest
Then chlorine
Then Bromine
Then fluorine
This is because fluorine and more electronegative elements form bonds that take much more energy to break. This factor is much more important than any effects due to electronegativity differences.
86
What is the effect of the nucleophile on the rate of nucleophilic substitution?
SN2 reactions are generally faster when ions are involved rather than neutral molecules. OH- ions will react faster than H2O molecules because the higher negative charge means it will be more strongly attracted to the C+ atom in the halogenoalkane.
But the rate of an SN1 reaction is not effected by changing the nucleophile because the nucleophile attacks only after the rate determining step.
87
Why do there need to be two nucleophilic substitution reactions?
SN2 is more favourable for primary halogenoalkanes because of steric effects.
In a tertiary halogenoalkane the alkyl groups surrounding the central carbon in a tertiary halogenoalkane make it much more difficult for the nucleophile to attack the central carbon atom.
If the OH- did get in, then there would be 5 large groups surrounding the central carbon atom in the transition state this would lead to instability and the activation energy would be very high.
SN1 is favourable for tertiary halogenoalkanes because methyl groups have an electro releasing effect so they are able to stabilise a postively charged carbon atom to which they are bonded. This means the positive charge is more spread out meaning the ion is more likely to be formed.
So SN2 rate of reaction is
primary>secondary>tertiary
So SN1 rate of reaction is
tertiary>secondary>primary
88
What effect can the solvent have on nucleophilic substitution?
Polar substances are better than non-polar solvents.
Then polar solvents can be divided into two groups:
Protic and aprotic:
Protic: Have a hydrogen attached to an N or an O, and so can participate in hydrogen bonding - examples are water, ethanol
Aprotic: Do not have a hydrogen attached to an N or an O, and so cannot participate in hydrogen bonding.
Protic solvents are very good at dissolving negative and postitive ions. Negative ions are solvated by interactions with the H atoms that are attached to the O or the N ions.
Aprotic solvents are not good at dissolving negative ions because they lack a very positive hydrogen.
- SN1 reactions are favoured by protic, polar solvents, this is because two ions are formed in the rate determining step of an SN1 reaction and these are stabilised by the presence of a protic solvent, which is able to solvate both ions effectively.
- SN2 reactions are favoured by a aprotic polar solvent because. The rate determining step relies on a negative nucleophile attacking the halogenoalkane, if this negative ion was surrounded by solvent molecules it would not be an effective nucleophile.
89
What solvents are best for SN2 reactions?
SN2 reactions are favoured by a aprotic polar solvent because. The rate determining step relies on a negative nucleophile attacking the halogenoalkane, if this negative ion was surrounded by solvent molecules it would not be an effective nucleophile.
90
What solvents are best for SN1 reactions?
SN1 reactions are favoured by protic, polar solvents, this is because two ions are formed in the rate determining step of an SN1 reaction and these are stabilised by the presence of a protic solvent, which is able to solvate both ions effectively.
91
What are protic solvents?
Protic: Have a hydrogen attached to an N or an O, and so can participate in hydrogen bonding - examples are water, ethanol
Protic solvents are very good at dissolving negative and postitive ions. Negative ions are solvated by interactions with the H atoms that are attached to the O or the N ions.
92
What aprotic solvents?
Aprotic: Do not have a hydrogen attached to an N or an O, and so cannot participate in hydrogen bonding.
Aprotic solvents are not good at dissolving negative ions because they lack a very positive hydrogen.
93
What is an electrophile?
An electrophile is an electron-deficient species (a positive charged ion or the positive end of a dipole) that is attracted to regions of relatively high electron density and accepts a pair of electrons to form a covalent bond - electrophiles are Lewis acid. Lewis acids are electron acceptors.
94
How do alkenes react with hydrogen halides?
They react by electrophilic addition reactions. The hydrogen on the hydrogen halide is partially positive because the molecule is polar. This hydrogen atom is attracted to the high electron density in the C=C bond in the alkene.
In the first stage a pair of electrons is donated from the pi bond to the hydrogen. At the same time the halogen hydrogen bond breaks and the pair of electrons from that bond goes to the halogen. The alkene can therefore be described as a Lewis base because it donates electrons and the halogen can be described as a Lewis Acid because it accepts electrons. The second carbon on the alkene has lost a pair of electrons and is therefore positive. It forms a carbocation and then the halogen is negative and so it is attracted and forms a bond.
The bond breaking is heterolytic fission because both electrons go to the halogen.
95
How does electrophilic addition reactions happen?
Between alkenes and hydrogenhalides.
The hydrogen on the hydrogen halide is partially positive because the molecule is polar. This hydrogen atom is attracted to the high electron density in the C=C bond in the alkene.
In the first stage a pair of electrons is donated from the pi bond to the hydrogen. At the same time the halogen hydrogen bond breaks and the pair of electrons from that bond goes to the halogen. The alkene can therefore be described as a Lewis base because it donates electrons and the halogen can be described as a Lewis Acid because it accepts electrons. The second carbon on the alkene has lost a pair of electrons and is therefore positive. It forms a carbocation and then the halogen is negative and so it is attracted and forms a bond.
The bond breaking is heterolytic fission because both electrons go to the halogen.
96
What is Markovnikov's rule?
Markovnikov's rule is that when H-X adds across the double bond of an alkene the H atom becomes attached to the C atom that has the larger number of H atoms already attached.
The explanation for this involves the stability of the intermediate carbocation. The hydrogen goes to the one with more hydrogens because then it forms a carbocation with the positive C in the middle, this is more stable than if the C+ was on the edge because alkyl groups have an electron releasing effect or otherwise called a positive inductive effect. So if the hydrogen goes with the Carbon that already has more C's it is at the edge, so the C+ is in the middle and the alkyl groups can stabilise that charge.
97
Why does the hydrogen, in addition reactions with an alkene, go to the carbon with more hydrogens?
Markovnikov's rule is that when H-X adds across the double bond of an alkene the H atom becomes attached to the C atom that has the larger number of H atoms already attached.
The explanation for this involves the stability of the intermediate carbocation. The hydrogen goes to the one with more hydrogens because then it forms a carbocation with the positive C in the middle, this is more stable than if the C+ was on the edge because alkyl groups have an electron releasing effect or otherwise called a positive inductive effect. So if the hydrogen goes with the Carbon that already has more C's it is at the edge, so the C+ is in the middle and the alkyl groups can stabilise that charge.
98
What will happen if Br-Cl adds on to an alkene?
This is the same reaction as a hydrogenhalide, the less electronegative of the halides acts as the hydrogen and goes to the carbon with already more hydrogens on there.
So in this instance, Br is less electronegative so following, Markovnikov's rule will attach to the Carbon further to the edge that has the more hydrogens already attached to it.
99
How does Bromine react with alkenes?
Although bromine molecules are not polar like hydrogenhalides, they can still react in a similar way.
As one of the bromines of the Br2 molecule approaches the double bond the electrons in the double bond repel the electrons in the 1st Br of the Br2 molecule and it acts like the hydrogen molecules.
100
What is the structure of benzene?
6 carbons, all with a hydrogen and a single bond and then a delocalised ring inside. The pi component of each two carbons is shared.
101
The nitration of benzene?
When benzene is heated with a mixture of concentrated nitric and sulphuric acids, nitrobenzene is formed.
C6H6 + HNO3 = C6H5NO
- This reaction occurs via electrophilic substitution.
The first stage in the reaction is the formation of the electrophile, this is the nitronium ion NO2+ which is formed when concentrated sulphuric acid reacts with concentrated nitric acid.
HNO3 + 2H2SO4 = NO2+ + H3O+ +2HSO4-
The second stage starts with a collision between a benzene molecule and the electrophile.
A pair of electrons from the benzene pi delocalised system is used to form a bond to the NO2+ electrophile. To form this bond, a carbon atom must use one of the orbitals that previously formed part of the pi delocalised system. The delocalised system can then only extend over the other five carbon rings. It has a partial ring.
There were originally six electrons in the pi delocalised system (one from each carbon atom) but two are now used to form the new C-N bond, which leaves only four electrons shared over five C atoms; so the ring has a positive charge.
In the third step the hydrogen on the carbon now with the NO2 is given to the HSO4- and it reforms H2SO4 and so it is unchanged in the reaction and can be regarded as a catalyst.
102
What do aldehydes reduce to?
Primary alcohols using NaBH4 and methanol
103
What is the catalyst for reduction of aldehydes?
NaBH4 and methanol
104
What do ketones reduce to?
Secondary alcohols using NaBH4 and methanol
105
What do carboxylic acids reduce to?
Primary alcohols,
using the stronger catalyst of
LiAlH4 in ethoxyene with H+ ions
106
What is the catalyst for the reduction of carboxylic acids?
LiAlH4 + ethoxyene with H+ ions.
107
How do you reduce nitrobenzene?
Nitrobenzene is reduced to phenylamine by heating with a reducing agent such as a mixture of tin and hydrochloric acid.
The catalysts are:
HCl
NaOH
Nitrobenzene goes from being a ring with a NO2 attached to being a ring with a NH2 attached, producing water too.
108
What are the catalysts for the reduction of nitrobenzene?
HCl
| NaOH
109
What are stereoisomers?
They have the same structural formula (the atoms are joined together in the same way - same connectivity) but the atoms are arranged differently in space.
110
What is cis-trans isomerism?
Two compounds have the same structural formula but the groups are arranged differently in space around a double bond or ring.
111
What is cis-trans isomerism possible?
Because free rotation is not possible around a C=C bond. The sigma bond does allow rotation but the pi one does not. The pi bond would have to be broken to allow rotation to occur and this takes a lot of energy.
112
What has to happen for a molecule to exhibit cis-trans isomerism?
For a molecule to exhibit cis-trans isomerism there must be two different groups on both sides of the double bond.
113
How do we know which side the trans or cis is and so how do we label a trans or cis isomer?
We use the E/Z naming system:
We look at each side of the C=C bond separately and assign a priority (1 or 2) to each of the atoms or groups attached. If the two groups with the highest priority are on the same side then the isomer is a Z isomer. If they are on opposite sides then it is a E isomer.
You compare each atom with the one opposite
For example if you have
H CH3
C=C
F Cl
H has a smaller atomic number than F, so F has the first priority. CH3 has an atomic number of 6 which is smaller than Cl so Cl is the first priority.
The highest priority are on the same side so it is a Z isomer.
DOUBLE BONDED ATOMS ACT AS TWO!
114
What is the E isomer?
The highest priority based on atomic number (highest is first) are on different sides.
115
What is the Z isomer?
The highest priority based on atomic number (highest is first) are the same side.
116
What can be different properties of cis-trans isomers?
Cis are usually polar because they have the same charges on the same side. Whereas trans are normally non-polar as they have different charges on the same side and they cancel.
117
What is isomerism like in cycloalkanes?
The condition for a cycloalkane to exhibit cis-trans isomerism is that at least two carbon atoms must have two different groups attached.
118
What are conformational isomers?
Conformational isomers are forms of the same molecule that have different conformations because of rotation about a sigma bond. These isomers all show the same molecule and these molecules in different spinning positions cannot be isolated because the barrier to rotation is so low. Different forms will exist simultaneously in a sample of the substance - and these forms will be constantly interconverting.
119
Compare conformational isomers and configurational isomers?
Conformational isomers ca be interconverted without breaking chemical bonds. Interconversion of configurational isomers requires the breaking and subsequent re-forming of chemical bonds.
120
What are optical isomers?
Optical isomerism is two isomers that are mirror reflections of one another and therefore are non-superimposible, you can only make one or two groups line up if you put different molecules on top of each other.
In order to exhibit optical isomerism there must be four different groups attached to a carbon atom. If it only had 3 it could be superimposable.
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What does something have to exhibit to be an optical isomer?
It has to have 4 different groups around the C atom. So have a chiral centre.
Not have a plane of symmetry.
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What is a chiral centre?
A carbon atom with four different groups around it.
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What is an enantiomer?
It is a single individual optical isomer of a compound.
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What is significant about a plane of symmetry down a molecule?
If a molecule has a plane of symmetry it will not exhibit optical isomerism.
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What do two enantiomers of an optically active compound do together?
Two enantiomers of an optically active compound have the property that they rotate plane polarised light in opposite directions.
Normal non-polarised light vibrates in all planes. If non-polarised light is passed through samples of the two isomers of butan-2-ol, we find that one of the isomers rotates the plane of polarised light to the right (clockwise) and the other isomer rotates the plane of the plane-polarised light to the life.
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What is a polarimeter?
A simple polarimeter consists of a source of light (usually a sodium lamp producing one specific wavelength) two polarising filters, a sample tube and a scale to measure the degree of rotation of the plane-polarised light.
