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Chemistry > Module 6: Organic Chemistry and Analysis > Flashcards

Module 6: Organic Chemistry and Analysis Flashcards

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1
Q

Kekulé model of benzene?

A

Six members ring of carbon atoms joined by alternate single and double bonds

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2
Q

Evidence to disprove kekulés model?

A
  1. Lack of reactivity of benzene - if the model was right, it would decolourise bromine in an electrophilic addition reaction.
  2. Length of C-C bonds in benzene - X-ray diffraction, measure lengths in a molecule. Benzene found to have 0.139nm length but single bond would be 0.153nm
  3. Hydrogenation enthalpies - would expect it to have hydrogenation enthalpy which is three times that of cyclohexane. But the actual is 152kjmol-1 less than expected.
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3
Q

Delocalised model of benzene?

A
  • each carbon atom uses 3/4 electrons in bonding to the two other carbons and one to the hydrogen atom.
  • each carbon has on electron in a p-orbital at right angles to the plane of the bonded carbon and hydrogen atom.
  • sideways overlap of P-orbitals, above and below plane of carbon atoms, form electron density ring above and below.
  • Overlapping creates system of pi bonds
  • The six electrons in this system are delocalised
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4
Q

When is phenyl used instead of benzene in naming?

A

When benzene ring is attached to alkyl chain with a functional group or to an alkyl chain which 7 or more carbon atoms.
Benzene becomes substituent.

E.g. phenylethanone, 2-phenyloctane

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5
Q

Type of reaction for benzene and it’s derivatives?

A

Electrophilic substitution

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6
Q

Nitration of benzene?

A

Benzene + HNO3
Catalysed by H2SO4 and heated to 50*C
Produce nitrobenzene and H2O

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7
Q

Halogenation of benzene?

A

Requires a halogen carrier (AlCl3, FeCl3 etc)

Get bromination and chlorination

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8
Q

Alkylation of benzene?

A

Substitution of hydrogen atom in benzene ring by a alkyl group.
Requires reacting with haloalkane In presence of AlCl3 halogen carrier catalyst

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9
Q

Acylation reaction of benzene?

A

Benzene reacts with acts chloride in presence of AlCl3 catalyst
Aromatic Ketone formed

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10
Q

Oxidation of Aldehydes?

A

Aldehydes can be oxidised to carboxylic acids.
Reflux with acidified dichromate VI ions, Cr2O7^2- / H+
(K2Cr2O7/H2SO4)

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11
Q

Oxidation of Ketones?

A

Do not undergo oxidation reactions

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12
Q

Carbonyl compounds?

A

Aldehyde
Ketone
Presence of C double bond O
Nucleophilic Addition reaction

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13
Q

Why is the carbonyl compounds reaction Nucleophilic addition?

A

It has a C=C bond which is polar
Oxygen is more electronegative than carbon, Electron density lies closer to the oxygen atom.
Creates delta + carbon and delta - oxygen
Nucleophile will be attracted to slightly positive carbon atom.

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14
Q

Aldehydes can be reduced yo what with the addition of what?

A

Aldehydes + 2[H] —-> Primary alcohol

Requires - NaBH4 / H2O

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15
Q

Ketones reduces to what?

A

Ketone + 2[H] —> secondary alcohol

Requires - NaBH4 / H2O

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16
Q

Carbonyl compounds with HCN?

A

Aldehyde or ketone + HCN —> hydroxynitrile

Requires - H2SO4/ NaCN

( CN- / H+ )

17
Q

How do you detect a carbonyl compound?

A

Add 2,4-dinitrophenylhydrazine
If no crystal form, add H2SO4
Yellow/orange precipitate forms which is 2,4-dinitrophenylhydrazone
Means there’s a ketone or aldehyde

18
Q

Distinguishing between aldehyde and ketone?

A

Tollens reagent
Add silver nitrate in aqueous ammonia
If silver mirror is formed then an aldehyde is present.

Ag+ + e- —> Ag (s) The silver mirror : reduction reaction

Aldehyde + [O] —> carboxylic acid : oxidation reaction

19
Q

What is a phenol?

A

Type of organic chemical containing a hydroxyl -OH functional group directly bonded to an aromatic ring.

20
Q

Why is phenol classified as a weak acid?

A

Phenol is less soluble in water than alcohols due to the presence of the non-polar benzene ring.
When dissolved in water, phenol partially dissociates to produce phenoxide ion and a proton.

21
Q

Phenol + Sodium Hydroxide

A

Salt - Sodium Phenoxide (O-Na+)
Water
Neutralisation reaction.

22
Q

Type of reactions Phenols undergo?

A

Electrophilic Substitution

23
Q

Bromination of Phenol?

A

Phenol + Bromine 3Br2 (bromine water) –> white precipitate of 2,4,6-tribromophenol.
This decolourises bromine water, orange to colourless.
Halogen carrier not required.
+3HBr

24
Q

Nitration of Phenol?

A

Phenol + dilute nitric acid (HNO3) –> mix of 2-nitrophenol and 4-nitrophenol
+H20

25
Q

Compare reactivity of Phenol to Benzene?

A

Phenol has increased reactivity. This is due to the lone pair of electrons from the oxygen p-orbital of the -OH group, being donated to the pi-system of phenol. The electron density of the benzene ring in phenol is increased, this attracts electrophiles more readily than benzene.

26
Q

What activates an aromatic ring?

A

NH2 (phenylamine) reacts more readily with electrophiles, directs second substituent to positions 2 and 4

27
Q

What deactivates and aromatic ring?

A

NO2 (nitrobenzene) reacts less readily with electrophiles, directs second substituent to position 3

28
Q

What is a carboxyl group?

A

Contains carbonyl group and hydroxyl group.

Carboxylic acid

29
Q

Solubility of carboxylic acids?

A

Both the C=O and O-H bonds are polar allowing carboxylic acids to form hydrogen bonds with water molecules.

30
Q

Qualitative analysis - test for Allene

A

Add bromine water

Decolourise from Orange to clear

31
Q

Qualitative analysis- test for haloalkane

A

Silver nitrate and ethanol
Warm to 50•C

Chloro white ppt
Bromo cream ppt
Iodo yellow ppt

32
Q

Qualitative analysis- carboxylic acid

A

Aqueous sodium carbonate

Effervescence

33
Q

Qualitative analysis- primary secondary alcohol and aldehyde

A

Acidified potassium dichromate warm

Orange to green

34
Q

What is the stationary phase in chromatography?

A

Does not move and is normally a solid or a liquid supported on a solid

35
Q

What is the mobile phase in chromatography?

A

Does move as is normally a liquid or a gas

36
Q

What is adsorption?

A

Process by which the solid silica holds the different substances in the mixture to its surface.

Separation is achieved by the relative adsorption of substances with the stationary phase.

37
Q

Interpreting TLC

A

Rf = distance moved by the component / distance moved by the solvent front

Chemistry (3 decks)

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