Module 4.2 - Alcohols, Haloalkanes and Analysis Flashcards
(42 cards)
1
Q
What is the general formula of alcohols?
A
CnH2n+1OH
2
Q
What are the 3 groups an alcohol can be classified as?
A
- primary alcohol
- secondary alcohol
- tertiary alcohol
3
Q
What is a primary alcohol?
A
Has the functional group attached to a carbon with no more than one alkyl group
4
Q
What is a seconday alcohol?
A
Has the functional group attached to a carbon atom with two alkyl groups
5
Q
What is a tertiary alcohol?
A
Has the functional group attached to a carbon atom with three alkyl groups
6
Q
Explain the trend in boiling point of alcohols as chain length increases.
A
- bp increases as chain length increases
- more areas of surface contact as higher Mr
- stronger induced dipole dipole intermolecular forces
- more energy to overcome these attractive forces so higher bp
7
Q
How does the boiling point of alcohols compare to their corresponding alkanes?
A
- bp of alcohols higher than corresponding alkanes
- H bonds between alcohol molecules
- H bonds stronger than London forxes
- so more energy needed to overcome
- so higher bp
8
Q
Describe the volatility of alcohols. How does this compare to the corresponding alkane?
A
- volatile substances evaporate at RTP
- volatility increases as bp decreases
- increases in chain length = decrease in volatility
- less volatile than corresponding alkane from H bonds
9
Q
Describe the solubility of alcohols in water, and how this is affected by an increase in chain length.
A
- water molecules are polar as is the alcohol functional group
- methanol, ethanol and propanol are soluble in water as form H bonds w water (sometimes described as miscibility)
- as chain length increases, solubility of alcohol decreases as the aliphatic chain can’t form H bond and this becomes the larger part of the molecule
10
Q
What are the products when an alcohol burns completely?
A
- carbon dioxide
- water
11
Q
Describe the distillation of primary alcohols.
A
- gently heat
- forms an aldehyde (partial oxidation)
- acidified potassium dichromate (VI) (K2Cr2O7, Cr2O72-) and sulphuric acid (H2SO4, H+) catalysts
C2H5OH + [O] –> C2H3HO + H2O
- aldehyde formed has to be distilled immediately to prevent further reaction
- colour change from orange to green
12
Q
Describe the reflux of primary alcohols.
A
- heat strongly
- forms an carboxylic acid (full oxidation)
- acidified potassium dichromate (VI) (K2Cr2O7, Cr2O72-) and sulphuric acid (H2SO4, H+) catalysts
C2H5OH + 2[O] –> C2H3HO + H2O
-colour change from orange to green
13
Q
Describe the reflux of secondary alcohols.
A
- heat strongly
- forms an ketone (full oxidation)
- acidified potassium dichromate (VI) (K2Cr2O7, Cr2O72-) and sulphuric acid (H2SO4, H+) catalysts
C2H5OH + [O] –> C2H3HO + H2O
-colour change from orange to green
14
Q
What is reflux?
A
The constant boiling and condensing of a reaction mixture. This ensures that the reactions goes to completion as fully as possible without losing reactants or products as vapour goes into the air
15
Q
How could you use a laboratory test to classify an unknown alcohol?
A
- heat the test alcohols w excess acidified potassium dichromate (VI) under reflux
- if no colour change to green it’s tertiary
- if colour change to green, distil product and test w Fehling’s solution or Tollen’s reagent
- second colour changes indicates test alcohol is secondary, but no second colour change indicates test alcohol is primary
16
Q
Describe the dehydration of an alcohol.
A
- water is lost from organic compound
- elimination reaction
- alcohols heated w strong acid (e.g. H2SO4) water elimintated to make an alkene
- OH group from one C and H from adjacent C lost to form water
- π bond forms between the 2 adjacent C atoms
17
Q
Describe the halide substitution as a halide ion reacts with an alcohol.
A
- ROH + HX –> RX + H2O
- forms a haloalkane
- reagant is a hydrogen halide (for bromide, NaBr used which forms HBr in H2SO4)
- conc acid catalyst e.g. conc H2SO4
- with iodide, H3PO4 used as H2SO4 oxidises I- to I2 so yield of iodoalkane would be v low
- mixture warmed to increase rate of reaction
18
Q
What are primary haloalkanes?
A
Halogen is at end of chain
19
Q
Describe and explain the reactivity of haloalkanes.
A
- large difference in electronegativity between carbon and halogens so polar bond (e- spend more time at X than C in C-X) so 𝛿+ on C atom and 𝛿- on halogen
- 𝛿+ C attacked by atoms, molecules or ions with partial or full negative charge (H2O, OH-, NH3) which are nucleophiles
20
Q
What factors affect the rate of reaction of haloalkanes?
A
- strength of C-X bond (going down group halogens get larger so bonding electrons are further form nucleus and have more shielding so lower mean bond enthalpy so C-X easier to break. Higher bond enthalpy = less likely halogen will react to nucleophilic substitution reactions)
- polarity of C-X bond (electronegativity decreases down group so polarity of C-X decreases down group. Most polar molecules are most likely to react)
- bond enthalpy is more important i.e. C-Cl is most polar but as C-Cl have highest bond enthalpy they have the slowest rate of hydrolysis
21
Q
What is a nucleophile?
A
Electron pair donor
22
Q
What is a nuclophilic substitution reaction?
A
A chemical reaction in which an atom or group of atoms is exchanged for a nucleophile
23
Q
How does a haloalkane undergo hydrolysis (by nucleophilic substitution)?
A
- water often used as reactant
- one of the H-O bonds undergoes heterolytic fission producing OH- nucleophile, or can come from an aqueous alkali as has solution of OH- ions. Aqueous alkali is faster than using water
- species attacks electron deficient C atom in haloalkane molecule
- once nucleophile bonded to haloalkane, C-X bond undergoes heterlytic fission to produce a halide ion
24
Q
How could you measure the rate of hydrolysis of the C-X bond in haloalkanes?
A
- rate of a chemical reaction is change in conc of a reactant or product over time
- one of the products of this hydrolysis is X- ion, halide ion
- reacts w silver nitrate to form the coloured silver halide ppt
- time to form coloured ppt used to determine rate of hydrolysis
25
How can chlorine radicals from CFCs catalyse the break down of ozone?
initiation: CF2Cl2 --\> •CF2Cl + Cl•
propagation: Cl• + O3 --\> •ClO + O2
•ClO + O3 --\> Cl• + 2O2 /// •ClO + O --\> Cl• + O2
overall: 2O3 --\> 3O2
26
How can nitrogen oxide catalyse the breakdown of the ozone layer?
•NO + O3 --\> •NO2 + O2
•NO2 + O --\> •NO + O2
Overall: O3 + O --\> 2O2
27
What equipment is needed for distillation?
- round bottom flask for mixture
- heating mantle
- thermometer holder
- thermometer
- condenser
- connector
- collecting flask
28
What equipment do you need for reflux?
- pear shaped flask
- heating mantle
- condenser
29
How can you separate immiscible liquids (e.g. an organic solvent and water)?
1. Mount an iron ring on a clamp stand and put a separating funnel through it. Remove stopper and ensure tap at bottom is closed
2. Pour mixture into funnel so it's more than half full. Wash out reaction vessel w water and add to separating funnel. Put stopper back
3. Take funnel out of ring and invert it. Open tap to equalise pressure. Close tap and gently shake mixture in funnel and equalise again as needed. Repeat until no longer hear a 'whistle'
4. Replace funnel in iron ring and give mixture time to separate. Remove stopper, put beaker under spout and open tap. Collect lower (water) layer in beaker, close tap. Aqueous layer can be discarded
5. Use a clean, dry beaker to open tap and collect desired organic product
6. Shake liquid w small amount of drying agent and pour final dry product to a clean, dry container
30
How would you use drying agents to dry chemicals?
1. Add a few spatulas of drying agent to organic product
2. If drying agent clumps together add more
3. When drying agent remains free moving organic product is dry
4. Use gravity filtration and collect filtrate, which is the dry organic product
31
Give one example of when redistillation may be used.
When ethanol is made by fermentation, distillation is used to remove ethanol from water, yeast and any remaining sugar. Distillate usually distilled again to purify the ethanol further
32
How would you test for an unsaturated hydrocarbon?
- add a few drops of bromine water to sample and shake
- positive result: bromine water decolorises
33
How do you test for a haloalkane?
- silver nitrate, ethanol and water
- white ppt indicates chloro-
- cream ppt indicates bromo-
- yellow ppt indicated iodo-
34
How do you test for carbonyls?
- acidified potassium dichromate (VI)
* ketone: no change, aldehyde: orange turns green
- Fehling's solution
* ketones: no change, aldehyde: dark red ppt
- Tollen's reagent
* ketones: no silver mirror, aldehydes: silver mirror
35
How do you test for a carboxylic acid?
- universal indicator/pH probe: shows pH of weak acid
- reactive metal e.g. Mg: effervescence (hydrogen)
- metal carbonate e.g. CaCO3: effervescence (carbon dioxide)
36
How do you test for an alcohol?
- warm w an equal volume of carboxylic acid and a few drops of H2SO4
- positive result: sweet smell of an ester after a short time
37
How does infrared spectroscopy affect the bonds in organic compounds?
causes the bonds to vibrate
38
What affects how much the bonds vibrate during infrared spectroscopy?
- bond strength
- bond length
- mass of each atom involved in the bond
39
What is a molecular ion, M+?
The positive ion formed in mass spectrometry when a molecule loses an electron
40
What does mass spectrometry do to molecules?
causes them to lose an electron forming positive ions. Excess energy from ionisation can be transferred to molecular ion, causing it to vibrate so bonds weak and fragments, forming fragment ions
41
Why is there a small peak above the molecular ion peak in mass spectra?
From the small amount of carbon-13
42
Do isomers have the same mass spectra?
No - branched isomers will have more CH3+ peaks than the straight chain isomer with the same Mr
