Functional group chemistry Flashcards
Why are alkanes unreactive?
- 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
How do alkanes undergo combustion?
- 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
What is free-radical substitution?
- Reaction in which halogens replaces a hydrogen
- Products: halogenoalkane + hydrogen halide
- Photochemical reaction in which ultraviolet (UV) radiation is present
Why is UV radiation required in free-radical substitution?
- 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
Explain initiation (1) and propagation (2) of the free-radical substitution.
- 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
Explain the termination (3) stage of free-radical substitution.
- 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
Why are alkenes more reactive than alkanes?
- 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
What type of addition reactions to alkenes undergo?
- Hydrogenation
- Hydration
- Halogenation
- Test for unsaturation
- Addition polymerisation
Explain hydrogenation. Why is it used in industry?
- 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)
Explain hydration.
- alkene + H2O (g) —> alcohol
- Sulfuric acid (H2SO4) or phosphoric acid (H3PO4) catalyst required
Explain halogenation.
- alkene + hydrogen halide —> halogenoalkane
- at r.t.p
OR - alkene + halogen —> dihalogenoalkane
Explain the test for unsaturation.
- 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
Explain addition polymerisation.
- 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)
What is the most common plastic polymer?
- Poly(chloroethene) or PVC, which is the most used plastic
- Synthesis is associated with toxins such as the by-product dioxin
Explain and be able to draw the monomer, repeating unit and polymer.
- 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
What determines the physical and chemical properties of alcohols? Why are alcohols soluble?
- 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
Do alcohols undergo combustion reactions?
- Complete combustion (excess O2)
- Incomplete combustion (lack of O2)
- Enthaply change of combustion
What classification of alcohols undergo oxidation reactions? How does oxidation occur?
- 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
What is partial oxidation? How is the product extracted?
- 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
What is complete oxidation? How is the product extracted?
- 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
What product is produced when secondary alcohols undergo oxidation? Why do tertiary alcohols not undergo oxidation?
- Ketone, extracted under reflux and oxidising agent
- Tertiary alcohols have no hydrogen bonded to the carbon that is bonded to the hydroxyl group
What are nucleophilic substitutions?
- Esterfication and condensation reactions with carboxylic acids
Explain esterfication.
- 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
How do you name esters?
- Alcohol name becomes alkyl group
- Carboxylic acid follows ending with -oate
- E.g. ethanol + ethanoic acid —> ethyl ethanoate
What are properties of esters?
- Volatile and have an odor
- Used in cosmetics, flavours and perfumes
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)
What type of reactions do halogenoalkanes undergo?
- (Nucleophilic) Substitution reactions
- Heterolytic bond fission
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
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
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)
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
What determines the stability and type of reactions benzene undergoes?
- Delocalised electrons within the structure ensure the stability and determine which reactions benzene undergoes
What type of reactions does benzene undergo?
- Substitution reactions with halogens
- Electrophilic substitution reactions
- Combustion reactions (same as in other hydrocarbons)
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
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)
What are electrophiles?
- Species that are electron deficient and have a (partial) positive charge
- They are attracted to the delocalised electrons
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
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
