Functional group chemistry Flashcards
(38 cards)
1
Q
Why are alkanes unreactive?
A
- C-H bonds are non-polar due to the small difference in electronegativity
- All C-C and C-H are covalent bonds, hence very strong and kinetically stable
2
Q
How do alkanes undergo combustion?
A
- Complete combustion (excess O2) produces CO2 and H2O
- Release large amounts of heat, hence used as fuels
OR - Incomplete combustion (limited supply of O2)
- Products: CO + H2O OR C (solid soot) + H2O
Be able to balance these equations
3
Q
What is free-radical substitution?
A
- Reaction in which halogens replaces a hydrogen
- Products: halogenoalkane + hydrogen halide
- Photochemical reaction in which ultraviolet (UV) radiation is present
4
Q
Why is UV radiation required in free-radical substitution?
A
- Used in homolytic bond fission to break the bonds between a halide molecule into two free-radicals
- They both have 7 electrons and hence are very reactive
- It is a chain reaction as the free-radicals initiate the reaction
5
Q
Explain initiation (1) and propagation (2) of the free-radical substitution.
A
- In initiation, 1 reaction, the homolytic bond fission occurs when the halide molecule is separated into two free radicals by UV radiation
- In propagation, 2 reactions, the alkane reacts with a free-radical. The free radical takes a hydrogen from the alkane. The product is a free radical alkane with 1 less H and a hydrogen halide
- The free radical alkane reacts with a new halogen molecule (2) to form the product in which a hydrogen is replaced by a halide + a free radical
- Free radical act as catalysts
6
Q
Explain the termination (3) stage of free-radical substitution.
A
- There are three possible outcomes:
2 free radicals react to form dihalide, free alkane radical and halide radical react to produce the wanted product or 2 alkane radicals react to produce a new alkane - Essentially unpaired electrons join together to get rid of the free-radicals
- This step stops the reaction
Check book
7
Q
Why are alkenes more reactive than alkanes?
A
- Unsaturated (double bond)
- Arranged in a trigonal planar (120°)
- More reactive than alkanes due to C=C as there is a region of high electron density at the double bond which is where also chemical reactions take place
8
Q
What type of addition reactions to alkenes undergo?
A
- Hydrogenation
- Hydration
- Halogenation
- Test for unsaturation
- Addition polymerisation
9
Q
Explain hydrogenation. Why is it used in industry?
A
- alkene + H2 (g) —> alkane
- Under high pressure and temperature (180°C)
- Nickle catalyst required
- Used in industry to make saturated compounds with high melting points (margarine)
- Results in trans fats (health concerns)
10
Q
Explain hydration.
A
- alkene + H2O (g) —> alcohol
- Sulfuric acid (H2SO4) or phosphoric acid (H3PO4) catalyst required
11
Q
Explain halogenation.
A
- alkene + hydrogen halide —> halogenoalkane
- at r.t.p
OR - alkene + halogen —> dihalogenoalkane
12
Q
Explain the test for unsaturation.
A
- Used to distinguish between alkenes and alkanes
- Halogen is added to both samples
- Bromine water is added
- Alkene decolourises the bromine, alkane mixture remains brown
- Brown —> colourless
13
Q
Explain addition polymerisation.
A
- A reaction in which monomers join together to form a long chain called polymer
- In a polymer there are repeating units
- n represents a large number, number of repeating units
- The chain is saturated
- Prefix poly- the suffix -ene and in brackets the monomer e.g. poly(ethene)
14
Q
What is the most common plastic polymer?
A
- Poly(chloroethene) or PVC, which is the most used plastic
- Synthesis is associated with toxins such as the by-product dioxin
15
Q
Explain and be able to draw the monomer, repeating unit and polymer.
A
- The monomer is the structure that is in the brackets
- Polymer is many monomers (without the double bond together)
- Repeating unit includes the brackets and the n
16
Q
What determines the physical and chemical properties of alcohols? Why are alcohols soluble?
A
- The functional group (hydroxyl group OH)
- Due to the H on the hydroxyl group, alcohols can form hydrogen bonds with other molecules and also with water
- Therefore it can dissolve in water
17
Q
Do alcohols undergo combustion reactions?
A
- Complete combustion (excess O2)
- Incomplete combustion (lack of O2)
- Enthaply change of combustion
18
Q
What classification of alcohols undergo oxidation reactions? How does oxidation occur?
A
- Primary and secondary
- Reaction with oxygen
- Due to the presence of hydroxyl group and an oxidising agent
E.g. potassium manganate (VII) from purple to colourless and acidified potassium dichromate (VI) from orange to green
19
Q
What is partial oxidation? How is the product extracted?
A
- Only with primary alcohols
- Oxidation with excess alcohol
- Result: aldehyde
- Aldehyde is removed by distillation as it has a lower boiling point (evaporates first) than the alcohol and carboxylic acid
- Normal distillation
20
Q
What is complete oxidation? How is the product extracted?
A
- Only with primary alcohols
- Result: carboxylic acid
- Conditions: heat under reflux and excess oxidising agent {O}
- To obtain the carboxylic acid, the mixture is heated under reflux
- Oxidising agent and alcohol vaporize and condense at the same time to complete complete oxidation
21
Q
What product is produced when secondary alcohols undergo oxidation? Why do tertiary alcohols not undergo oxidation?
A
- Ketone, extracted under reflux and oxidising agent
- Tertiary alcohols have no hydrogen bonded to the carbon that is bonded to the hydroxyl group
22
Q
What are nucleophilic substitutions?
A
- Esterfication and condensation reactions with carboxylic acids
23
Q
Explain esterfication.
A
- Reaction between alcohols and carboxylic acids in condensation reactions
- Product: ester and H2O
- Reversible reaction (equilibrium)
- Requires a strong acid catalyst e.g. sulfuric acid
24
Q
How do you name esters?
A
- Alcohol name becomes alkyl group
- Carboxylic acid follows ending with -oate
- E.g. ethanol + ethanoic acid —> ethyl ethanoate
25
What are properties of esters?
- Volatile and have an odor
| - Used in cosmetics, flavours and perfumes
26
What are halogenoalkanes and what are their properties?
- Compounds in which a hydrogen is replaced by a halogen atom
- General formula: CnH2n+1X
- More reactive than alkanes
- Oily liquids and do not mix with water (inability to form hydrogen bonds)
27
What type of reactions do halogenoalkanes undergo?
- (Nucleophilic) Substitution reactions
| - Heterolytic bond fission
28
Why do subsitution reactions work in halogenoalkanes?
- Group or one atom is replaced by another atom or group
- The carbon-halogen bond is polar (due to the difference in electronegativity)
- The bond is electro-deficient and can be attacked by nucleophiles
29
What are nucleophiles?
- Electron-rich species that contain lone pair of electrons and are attracted to regions of positive charge
- They are attracted to the carbon-halogen bond for instance
- They donate lone pair of electron to the electron-deficient carbon
30
What is heterolytic bond fission?
- Carbon-halogen bond breaks, one atom takes both of the bonding electrons which forms oppositely charged ions (halogen becomes negative)
31
How does nucleophilic substitution work?
- A hydroxide ion (OH-) is substituted from a strong alkali (e.g. potassium or sodium hydroxide) into the halogenoalkane
- The OH- is the nucleophile and replaces the halogen to produce an alcohol
- Due to the heterolytic bond fission, the halogen of the halogenoalkane takes both bonding electrons and reacts with the alkali from the alkali solution
- Conditions: heat and dilute solution of a strong alkali
e. g. CH3CH2Cl + NaOH --> CH3CH2OH + NaCl
32
What determines the stability and type of reactions benzene undergoes?
- Delocalised electrons within the structure ensure the stability and determine which reactions benzene undergoes
33
What type of reactions does benzene undergo?
- Substitution reactions with halogens
- Electrophilic substitution reactions
- Combustion reactions (same as in other hydrocarbons)
34
What is a substitution reaction in benzene?
- A hydrogen is replaced by an electrophile, this does not disrupt the ring structure which makes it more energetically favourable
- Benzene does not undergo addition reactions as that would disrupt the structure
35
What do delocalised electrons do in substitution reactions?
- They provide a region of electron density, hence attract species called electrophiles where they react (in the inside ring of benzene)
36
What are electrophiles?
- Species that are electron deficient and have a (partial) positive charge
- They are attracted to the delocalised electrons
37
How does benzene undergo electrophilic substitution reactions?
- The electrophile is a cation e.g. nitronium ion from a compound
- Product e.g. nitrobenzene and H2O
- Requires concentrated sulfuric acid as a catalyst
38
How does benzene undergo substitution reactions with halogens?
- The electrophile is a halogen e.g. Cl2
- Product: halogenbenzene and hydrogen halide
- Catalyst is aluminum halogen
