ORGANIC waffle Flashcards
1
Q
Explain the energetic stability of benzene? in comparison to kekule?
A
-benzene is 152 kJ/mol more energetically stable than kekule’s molecular structure. This is accounted for by the delocalisation.
The more spread the electrons are the more stable the molecule becomes.
-(extra stability of benzene is often referred to as ‘delocalisation energy’)
2
Q
What are the properties of benzene?
A
- colourless, flammable liquid with a sweet odour.
- boiling point if 80 degrees.
3
Q
Why isn’t kekule structure correct?
A
-if benzene contains 3 C=C bonds it should readily decolourise bromine in an addition reaction however it instead reacts in a substitution reaction.
-C4H4Br2 (dibromobenzene) should have 4 isomers according to kekule structure however only 3 isomers exist.
-the bond lengths (C-C & C=C) are the same length and perhaps also intermediate in character between C=C and C-C
- actual benzene is also a lot more stable than kekule structure.
Delta H for hydrogenation of benzene is less negative than expected
4
Q
Explain the shape of the benzene molecule?
A
- benzene is a planar regular hexagon, with bond angles of 120.
- it is a regular hexagon because all the bonds are identical. The delocalisation of the electrons means that there aren’t alternating double and single bonds.
- it is a planar structure because that is the only way that p orbital can overlap sideways to give the delocalised pi system
5
Q
What happens to the 4th electron in the p orbital of each C atom in benzene?
A
-it delocalises to form rings of electron density above and below the hexagon, forming rings of delocalised electron density above/below the hexagon
6
Q
Why does benzene have a relatively high melting point?
A
Close packing of flat hexagonal molecules when solid.
7
Q
What is resonance?
A
- resonance suggests the 2 structures rapidly alternate between 2 forms.
- the resonance explanation suggests that benzene is in such rapid equilibrium between the two forms;we detect a ‘blurred’ combination of the two forms.
- therefore the electrons from the double bonds are drawn as a circle shared equally between carbon atoms.
8
Q
What does aromatic mean?
A
-describes the bonding in a compound - delocalised electrons forming pi bonding a hydrocarbon ring.
9
Q
Dangers of benzene?
A
-It is carcinogen
10
Q
What does chiral mean?
A
-refers to an atom in a molecule that allows it to exist as non-superimposable forms.
11
Q
What does the term asymmetric mean?
A
-refers to a carbon atom in a molecule that is joined to four different atoms or groups.
12
Q
What does the term enantiomers mean?
A
-isomers that are related as object and mirror image
13
Q
chiral?
A
-an atom in a molecule that allows it to exist as non-superimposable forms.
14
Q
asymmetric?
A
refers to a carbon atom in a molecule that is joined to 4 different groups
15
Q
enantiomers?
A
are isomers that are related as object and mirror image
16
Q
similarities and differences between 2 optical isomers (enantiomers)?
A
- same atoms and bonds, but they are non-superimposable mirror images of one another.(not necessarily identical chemical properties)
- differ in the way they rotate plane polarised light-rotate plane of polarisation by the same angle in different directions.
17
Q
what is a polarimeter?
A
-apparatus used to measure the angle of rotation
18
Q
what is a racemic mixture?
A
is an equimolar mixture of two enantiomers that has no optical activity.
19
Q
what effect does the racemic mixture have on plane polarised light?
A
none as the rotation by each enantiomer cancels out to nothing
20
Q
what effect does + isomer have on plane polarised light?
A
rotates it X degrees clockwise
21
Q
what is an SN2 mechanism?
A
-its when theres two species involved in the rate-determining step.
22
Q
what kind of inter molecular forces do molecules with carbonyl group have?
A
permanent dipole (due to polar C=O) + london forces
23
Q
explain the solubility of carbonyls in water?
A
the smaller aldehydes and ketones are freely soluble in water - as they can hydrogen bond with water molecules
24
Q
why do the solubility of carbonyls fall as chain length increases?
A
-as the chain length increases the hydrocarbon ‘tails’ of the molecules start to get in the way, by forcing themselves between water molecules, they break the relatively strong hydrogen bonds between water molecules without replacing them thus making the reaction less profitable energetically.
25
why are ketones resistant to oxidation?
- ketones don't have the hydrogen atom attached to the carbonyl group.
- ketones only oxidised by powerful agents that can break C-C bonds.
26
which bond in carbonyl compounds is usually involved in reaction? why?
C=O, due to the polarity of the bond
27
outline how a polariser works
-when monochromatic light passes through the polariser it absorbs all oscillation except those in one plane
28
what reagent is used to reduce carbonyls?
lithium tetrahydridoaluminate
29
conditions for the reduction of carbonyls?
-both carbonyl compound and reducing agent dissolved in dry ether
30
Why are aldehydes easy to oxidise?
They have an hydrogen atom attached to the carbon-oxygen double bond in the aldehyde.
31
What is formed when an aldehyde is oxidised in acidic conditions?
Carboxylic acid
32
Whwhat is formed when a n ester is oxidised in alkali conditions?
A salt of carboxylic acid is formed
33
Explain the reaction of aldehydes with acidified potassium dichromate?
- orange solution turns green. (+ve result)
| - the orange dichromate (VI) ions have been reduced to green chromium (III) ions by the aldehyde.
34
Reaction between tollens reagent and aldehyde ?
| +ve result?
-the colourless solution produces a Grey precipitate of silver, or a silver mirror on the test tube.
35
What is the tollens reagent and how is the test carried out?
Tollens' reagent contains the diamminesilver(I) ion, [Ag(NH3)2]+.
To carry out the test, you add a few drops of the aldehyde or ketone to the freshly prepared reagent, and warm gently in a hot water bath for a few minutes.
36
Fehlings solution + aldehyde?
The blue solution produced a dark red precipitate of copper (I) oxide.
Aldehydes reduce the complexed copper (II) ion to copper (I) oxide.
37
What is the iodoform (triiodomethane)reaction used for?
-it is used to test for the presence of a CH3CO group in aldehydes, ketones and some alcohols.
38
What is the positive result for the iodoform reaction?
- a pale yellow precipitate of triiodomethane Or CHI3
39
Reagent for the iodoform reactions?
Iodine in alkaline conditions
40
Which is the only aldehyde that will give a positive result for the iodoform test?
Ethanal
41
What is a qualitative test for the presence of the carbon-oxygen double bond in carbonyls?
-Brady’s reagent 2,4-DNP
42
What is the positive result for the 2,4 - DNP test?
Bright orange precipitate
43
why is the 2,4-DNP test a condensation reaction?
-a condensation reaction is one which 2 molecules join together with the loss of a small molecule in this case water.
44
explain the mechanism for the 2,4 DNP reaction?
-its a nucleophilic addition-elimination reaction. the 2,4 DNP first adds across the carbon-oxygen double bond to give an intermediate compound which then loses a molecule of water
45
how is the 2,4 DNP used to identify the specific aldehyde or ketone?
- the precipitate is filtered and washed and then recrystallised.
- the crystals are dissolved in the minimum quantity of hot solvent. as the solution cools the crystals are filtered and dried (making them pure)
- if you find the melting point of the crystals you can compare them with all the common aldehydes and find out what it was initally.
46
what is nucleophilic addition?
-mechanism in which a molecule containing 2 atoms or groups is added across a polar double bond, and the attacking species in the first step is a nucleophile.
47
what are derivatives?
-compounds formed from other compounds
48
how can optical activity be used as evidence for the nucleophilic addition mechanism?
one with HCN
- -the arrangement of the two atoms or groups joined to C=O is planar, which means that in step 1 of the reaction there is an equal chance of the cyanide ion attacking from each side of the plane.
- so even though the reaction contain a asymmetric carbon, there are equal amount of both enantiomers thus forming a racemic mixture with no optical activity.
49
explain the reaction between carbonyls with hydrogen cyanide (nucleophilic addition)?
- reagent is HCN and the reaction is usually carried out in an aqueous solution containing KCN
- addition involves a hydrogen atom attaching to the oxygen atom of the carbonyl group and a cyanide group attaching to the carbon atom of the carbonyl group.
- hydroxynitrile is formed
50
why is the addition of HCN important in the nucleophilic addition reaction?
-increases the length of the carbon chain by 1 carbon atom which is very useful.
51
nucelophilic addition with HCN at very high pH? ?
-few H+, step 2 slows down ; less HCN
52
nucelophilic addition with HCN at very low pH? ?
-high H+ , low CN-, step 1 slows dwon
53
the reason behind the high boiling points of carboxylic acids?
- hydrogen bonding but operating in a different way
- 1 hydrogen bond can occur between 2 molecules of acid to produce a dimer.
- this doubles the size of the molecules and so increases the van der waals forces between one of these dimers and its neighbours.
54
reduction of carboxylic acids?
- reduced to an alcohol (happens in 2 steps via aldehyde)
| - reducing agent LiAlH4 in a solvent of dry ether.
55
product of the reduction of butanoic acid ?
butan-1-ol
56
How are carboxylic acids formed?
- oxidation of primary alcohols or aldehydes
| - hydrolysis on nitriles- either by heating under reflux or in acidic or alkaline solution
57
Explain the neutralisation reactions of carboxylic acids?
- carboxylic acid are neutralised by mixing with aqueous alkali.
- products are carboxylate salts that have a wide range of uses.
58
Explain the reaction of carboxylic acids + carbonates?
- carboxylate salt is formed along with carbon dioxide and water.
- this test can be used as a test for carboxylic acid as CO2 is formed.
59
Explain the halogenation of carboxylic acids?
- the OH group is usually replaced using PCl5.
- functional group becomes COCl
- it must be anhydrous conditions as both the reagent and product can react with water.
60
How can you get the acyl chloride from the halogenation of carboxylic acids?
-it can be separated by fractional distillation
61
Explain the esterification of carboxylic acids?
- carboxylic acid is mixed with an alcohol and small amount of acid catalyst. +heated
- reaction is reversible
- water is also produced
62
What acyl chlorides?
-acid derivatives
-general formula - ROCl
2 functional groups sharing the same carbon atom
63
Properties of acyl chlorides? (Appearance, solubility and bp)
- colourless liquid has a strong smell
- they don’t dissolve in water
- b.p are quite low ; higher than alkanes of similar length due to dipole -dipole interactions.
64
Reaction of water with acyl chlorides?
-acyl chlorides react vigorously with cold water, forming a carboxylic acid and releasing hydrogen chloride gas
-
65
Reaction of alcohols + acyl chlorides?
-forms an ester , at room temperature and is irreversible therefore better than the alcohol + carboxylic acid reaction.
-
66
Reaction of acyl chlorides with conc ammonia solutions?
- react readily
- the H if the second reactant combined with Cl to form HCl, and the other part of the reactant (NH2) joins the carbonyl group
- the NH2 joined to the carbonyl group produces an amide.
67
Explain the reaction between acyl chlorides and amines?
- primary amine - RNH2
- the H of the second reactant combines to form HCl and the other part of the reactant (RNH) joins the carbonyl group.
- product is a N-substituted amide.
68
How do you name esters?
- the first part comes from the alkyl group joined to O
| - the second word comes from the alkyl group joined to C
69
What are the physical properties of esters?
- esters are colourless liquids with relatively low boiling and melting points. (No hydrogen bonding)
- soluble in water as they can hydrogen bond with it.
- they have a good smell e.g.- pentyl ethanoate in pears.
70
Explain the hydrolysis of esters in acidic solution?
- reaction is catalysed by acid , so the ester is heated under reflux
- a carboxylic acid + alcohol is formed
- the reaction is reversible therefore slow and will not go to completion
71
How would you make the hydrolysis of esters in acidic condition as complete as possible?
- use excess water ( by using excess acid since the water comes from there)
72
Over where?
OVER THERE
73
Explain the hydrolysis of esters in alkaline conditions?
-ester is heated under reflux with Alkali e,g, NaOH
-a carboxylate salt is produced but the reaction goes to completion therefore it’s advantageous
.
74
How would you convert the carboxylate salt made in the hydrolysis to carboxylic acid?
- Add an excess of a dilute strong acid.
- By doing this, the mixture is flooded with H+ ions
- Carboxylate ions combine with these ions to form a carboxylic acid.
75
What is saponification?
- soap-making
| - it is an example of the alkaline hydrolysis of esters.
76
How does saponification reaction work?
-vegetable oils contain large quantities of triglycerides which are triesters - which consist of a large ester molecule that can be hydrolysed to one alcohol and three carboxylic acid molecules.
77
What are some uses of glycerol?
Uses as a food sweetener and in toothpaste
78
Explain the importance of optical isomers in biochemistry?
- optical isomers show identical chemical properties in most reactions.
- however certain biochemical processes require the molecule to have a specific orientation of groups.
- many drugs and enzymes are chiral and so only one of the optical isomers will be able to interact effectively with the targets molecule in the body.
79
What is condensation polymerisation?
-the formation of a polymer usually by the reaction of 2 different monomer in which a small molecule is also produced
80
How do you name a carboxylate salt?
- carboxylate salt are named with the cation first and then the suffix.
81
How do you name a carboxylate salt?
- carboxylate salt are named with the cation first and then the suffix.
82
describe a simple chemical test to differentiate a carboxylic acid from phenol and an alcohol?
-add a carbonate, the carboxylic acid will react the others wont.
83
what are the differences between a Carbonyl C=O bond and an alkene C=C bond?
C=O - polar bond so has permanent dipole across the bond.
- double bond provides a region of high electron density
C=O is prone to additions and nucleophillic attack because or carbon's positive charge and oxygen's negative charge.
C=C - double bond provides a region of high electron density. the pi bond is vulnerable to attack from electrophiles
84
compare the kekule and delocalised models for benzene reference the p orbitals and pi bonds?
kekule: sideways double overlap of p orbitals between two carbon atom
- 3 x pi bonds
-localised electron density in pi bond
-both C-C and C=C bonds present
Delocalised: sideways double overlap of p-orbitals between adjacent carbon atoms both above and below the plane of the ring
- 1 x pi bonding region
-delocalised lower electron density in pi region
-equal C-C bondd present
85
in a alkylation reaction when CH3Cl why is electrophile CH3+ formed rather than Cl- ?
Cl is more electronegative than C so when bond breaks by heterolytic fission both electrons go to the Cl
86
why is phenol more reactive than benzene?
- lone pair of electrons form O p-orbital is partially delocalised into the ring (pi region)
- pi region has higher electron density; pi region is activated
- pi region more able to polarise electrophiles and attracts them more strongly
- phenol is less stable --> more reactive
87
explain the reaction and conditions for the hydrogenation of benzene?
- mixing benzene with hydrogen and heating under pressure with a nickel catalyst.
- cyclohexane is formed
- basically adding hydrogen all the way around the benzene ring , these reaction destroy the electron delocalisation in the original benzene ring, since those electrons are being used in the new reaction
88
explain the halogenation reactions with benzene? condition etc
- halogen and benzene heated under reflux in the presence of a halogen carrier catalyst
- halogen carrier is a metal-halogen compound ( aluminium bromide or chloride)
