Unit 2.1 Basic Concepts And Hydrocarbons Flashcards
Why can carbon form so many compounds?
A carbon atom can:
- Form bonds with other carbon atoms to make chains and rings
- Form a single, double or triple bond to another carbon atom
- Bond with atoms of other elements
Definition of hydrocarbon
Organic compounds that contain carbon and hydrogen only
Definition of saturated hydrocarbon
A hydrocarbon with single bonds only
Definition of unsaturated hydrocarbon
A hydrocarbon containing carbon-to-carbon multiple bonds
Definition of aliphatic hydrocarbon
A hydrocarbon with carbon atoms joined together in straight (unbranched) or branched chains
Definition of alicyclic hydrocarbon
A hydrocarbon with carbon atoms joined together in a ring structure
Definition of functional group
The part of the organic molecule responsible for its chemical reactions
Definition of homologous series
A series of organic compounds with the same functional group but with each successive member differing by CH2
Definition of alkane
Saturated straight-chained hydrocarbon with single C-C bonds only
Definition of alkyl group
An alkane with a hydrogen atom removed
What is the suffix (ending of the word) and formula for an aldehyde?
Suffix: -al
Formula: -CHO
What is the suffix and formula for a ketone?
Suffix: -one
Formula: C-CO-C
What is the suffix and formula for a carboxylic acid?
Suffix: -oic acid
Formula: -COOH
Definition of general formula
The simplest algebraic formula of a member of a homologous series
What does a displayed formula do?
Shows the relative positioning of all the atoms in a molecule and the bonds between them
What does a structural formula do?
Shows the minimal detail for the arrangement of atoms in a molecule
What is the structural formula of propane?
CH3CH2CH3
What is a skeletal formula?
A simplified organic formula, with hydrogen atoms removed from alkyl chains, leaving just a carbon skeleton and associated functional groups
Definition of structural isomers
Molecules with the same molecular formula but with different structural arrangements of atoms
Name 2 structural isomers of C4H10
E.g. Butane, 2-methylpropane
Definition of stereoisomers
Compounds with the same structural formula, but with a different arrangement in space
Definition of E/Z isomerism
A type of stereoisomerism in which different groups attached to each carbon of a C=C double bond may be arranged differently in space because of the restricted rotation of the C=C bond
What are the 2 criteria that a molecule must satisfy to have E/Z isomerism?
- A carbon-carbon double bond must be present
- Each carbon in the double bond must be attached to 2 different groups
Definition of cis-trans isomerism
A special type of E/Z isomerism in which there is a non-hydrogen group and hydrogen on each C of a C=C double bond
What is homolytic fission?
The breaking of a covalent bond, with one of the bonded electrons going to each going to each atom, forming 2 radicals
What is heterolytic fission?
The breaking of a covalent bond with both of the bonded electrons going to one of the atoms, forming a cation and an anion
What is a radical?
A species with an unpaired electron
What is a nucleophile?
An atom that is attracted to an electron-deficient centre or atom, where it donates a pair of electrons to form a new covalent bond
What is an electrophile?
An atom that is attracted to an electron-rich centre or atom, where it accepts a pair of electrons to form a new covalent bond
What is an addition reaction?
A reaction in which a reactant is added to an unsaturated molecule to make a saturated molecule
2 products —-> 1 product
What is a substitution reaction?
A reaction in which an atom or group of atoms is replaced with a different atom or group of atoms
2 reactants —–> 2 products
What is an elimination reaction?
The removal of a molecule from a saturated molecule to make an unsaturated molecule
1 reactant —-> 2 products
What is fractional distillation?
A fossil fuel made from naturally decaying plants and animals that once lived in ancient seas, millions of years ago
It is a mixture of different hydrocarbons
How is crude oil refined?
By fractional distillation
How does fractional distillation work?
- It takes place in a fractionating column
- The crude oil is first vaporised by heating and is then passed into the fractionating column
- The gases pass up the column through a series of bubble caps
- The column is hotter at the bottom than at the top
- When the gases reach a temp that is lower than their boiling point, they condense into a liquid
- The liquid fractions are then tapped off into storage containers
What condenses near the top of the column, and why?
Short-chained hydrocarbons; lower boiling points
What effect does increasing the chain length of alkanes have on the boiling point?
- As the chain length increases, the boiling point increases because the intermolecular forces between the molecules get stronger
- There are more points of contact between the molecules, so there are more van der Waals’ forces between the molecules
- So more energy is needed (and therefore a higher temp) to separate the molecules
What effect does branching have on the boiling point of an alkane?
A branched isomer has a lower boiling point than the unbranched isomer with the same molecular mass:
- The branched alkane has fewer points of contact between molecules
- So there are fewer van der Waals’ forces between the molecules
- Branched molecules cannot get as close to each other as unbranched ones
- This reduces the intermolecular forces between the molecules
So less energy is required to separate the molecules
What happens when short-chain alkanes are burnt in a plentiful supply of oxygen?
They form carbon dioxide and water
What happens when alkanes are burnt in a limited supply of oxygen?
- They undergo incomplete combustion
- Form carbon monoxide and water
What is cracking?
The break down of long-chained saturated hydrocarbons to form a mixture of shorter-chained alkanes and alkenes
What is isomerisation?
- A form of cracking
- Unbranched alkanes are converted into branched alkanes
What is reforming?
- A form of cracking
- Aliphatic (straight-chain or branched) hydrocarbons are converted into cyclic or aromatic hydrocarbons
What can crude oil be used for? Give 3 examples.
- As a source of fuel
- Plastics
- Pharmaceuticals
- Cosmetics
- To make roads
- Dyes
- Inks
Why can burning hydrocarbons be bad for the environment?
Releases atmospheric pollutants, such as:
- Carbon monoxide
- Carbon dioxide
- Nitrogen oxides
- Sulfur dioxide
The release of these gases could be causing global warming
What fuels could be used instead of hydrocarbons from fossil fuel?
Biofuel: - derived from recently living material (e.g. Plants, animal waste)
Ethanol: - made by fermenting sugar and other carbohydrates
- burns efficiently in a plentiful supply of oxygen, to give CO2 and oxygen
Bioethanol: - A fuel oil derived from natural sources such as plants
- Can be used 100% pure, but it is usually blended with normal diesel
What is radical substitution?
A type of substitution reaction in which a radical replaces a different atom or group of atoms
What are alkenes?
Unsaturated hydrocarbons with at least 1 C=C double bond
What happens in the formation of a double bond?
- A σ-bond is formed directly between 2 carbon atoms by the overlap of orbitals. Each carbon atom contributes 1 electron to the electron pair in the σ-bond
- A π-bond is formed above and below the plane of the carbon atoms, by sideways overlap of p-orbitals. Each carbon atom contributes 1 electron from a p-orbital to the electron pair in the π-bond
What does the π-bond in an alkene do?
- It fixes the carbon atoms in position, at either end of the double bond
- This prevents any rotation of the bond
In alkenes, how is each carbon involved in the double bond?
- 3 of its electrons are used in the formation of 3 σ-bonds
- 1 of its electrons is used in the formation of a π-bond
What is the shape of an alkene molecule?
- 3 regions of electron density surround each carbon atom in the double bond
- Pairs of electrons repel each other as far apart as possible to minimise repulsions
- The electron pairs repel each other to give a trigonal planar shape, with bond angles of approx. 120°
Why are alkenes more reactive than alkanes?
Because of the C=C double bond: - A double bond is stronger than a single bond - A π-bond is weaker than a σ-bond When an alkene reacts: - The π-bond breaks - The σ-bond remains intact
What type of bond is between the 2 carbon atoms in an alkane?
A σ-bond
What type of reactions do alkenes typically take part in?
Addition reactions
What happens to an alkene in an addition reaction?
- A small molecule is added across the double bond, causing the π-bond to break
- 2 reactant molecules react together to form 1 product molecule
- An unsaturated alkene reacts and forms a saturated product
What happens when hydrogen is added to an alkene?
- The hydrogen adds across the double bond and an alkane is formed
- This is a reduction reaction
- It is sometimes known as hydrogenation
What conditions are required for the addition of hydrogen to an alkene?
- A temp of 150°C
- A nickel catalyst
What happens when halogens are added to alkenes?
- The halogen adds across the double bind to give a di-substituted halogenoalkane
- The reaction is sometimes known as halogenation
It is a rapid reaction at room temperature
What happens when a hydrogen halide is added to an alkene?
- It adds across the double bond
- Produces a halogenoalkane
What happens when steam is added to an alkene?
An alcohol is produced
What conditions are required for the addition of steam to an alkene?
- High temperature and pressure
- Phosphoric acid catalyst
Why is the double bond in alkenes a region of high electron density?
Because of the π-electrons in the double bond
What happens when hydrogen bromide is added to an alkene? (Electrophilic addition)
- Bromine is more electronegative than hydrogen, causing a dipole
- The electron pair in the π-bond is attracted to the slightly positive hydrogen atom, causing the double bond to break
- A new bond forms between 1 of the carbon atoms and the hydrogen atom
- The H-Br bond breaks by heterolytic fission, with the electron pair going to the bromine
- A bromide ion and a carbocation are formed
- The carbocation is unstable, so it quickly reacts with the bromide ion to form the organic product
How is bromine added to an alkene?
By electrophilic addition
What is a polymer?
A long molecular chain built up from monomer units
What is a monomer?
A small molecule that combines with many other monomers to form a polymer
How can vegetable oil be hardened to make margarine?
By hydrogenation:
- Adds hydrogen molecules across the double bonds in an addition reaction
- Alters the individual molecule in the oil in such a way that the oil partially solidifies and hardens
By adding hydrogen bonds across different numbers of the many double bonds, you can harden the oil by different amounts
Why is partial hydrogenation of unsaturated fats thought to be unhealthy?
- Can transform some cis double bonds into trans double bonds as a by-product
- These trans fats are thought to be bad for health
What is addition polymerisation?
The process in which unsaturated alkene molecules (monomers) add on to a growing polymer chain one at a time, to form a very long saturated molecular chain (the addition polymer)
What are the 2 different industrial processes in which addition polymerisation is carried out?
Radical polymerisation
The Ziegler-Natta process
What conditions are required for radical polymerisation? What does it produce?
- Temp of 200°C
- Very high pressure
Leads to branching of the polymer chain and the production of polymer mixtures
What conditions are required for the Ziegler-Natta process?
- Temp of 60°C
- Specialist catalysts such as TiCl3 and Al(C2H5)2Cl
What happens in the Ziegler-Natta process?
- The alkene is passed over the catalyst
- The conversion is low
- Any unreacted alkane is recycled and passed over the catalyst repeatedly
What is a repeat unit?
- A specific arrangement of atoms that occurs in the structure over and over again
- They are included in brackets, outside of which is the symbol ‘n’
Why are addition polymers difficult to dispose of?
- Don’t usually react with the substances placed inside them
- Fairly durable and do not break down naturally
Why is it important to recycle polymers?
- They are non-biodegradable
- They are rapidly filling up landfill space
How are polymers recycled?
-The different types of plastics are separated from each other
Reclamation:
- They are mechanically chopped into small flakes
- Then washed to remove any impurities
These flakes can be melted + remoulded to form new materials
How can polymer waste be used?
- Polymers can be burnt under controlled conditions to produce heat energy, which can be harnessed to make electricity
- Polymers can be converted into synthesis gas (a mixture of hydrogen and carbon monoxide). Hydrocarbons and synthesis gas can be used as a chemical feedstock for conversion into useful products or as a fuel at oil refineries
- PVC can be recycled by dissolving it in solvents (to separate it from other scrap) then recovering it from the solvent by precipitation
What are bioplastics?
- Plastics that are biodegradable or compostable
- They break down as a result of bacterial activity
- Produce carbon dioxide and water when they degrade
What are bioplastics made from?
Renewable raw materials such as:
- Starch
- Maize
- Cellulose
- Lactic acid
Describe how hydrogen can be used in the manufacture of margarine
Unstaurated oils reacted with hydrogen in the prescence of a nickel catalyst
What does ‘curly arrow’ represent?
The movements of electron pairs
