Alkanes and Alkenes Flashcards
General formula
The simplest algebraic formula of a member of a homologous series e.g. for an alkane: CnH2n+2
Structural formula
The arrangement of atoms in a molecule for ethanol it would be CH3CH2OH
Displayed formula
The relative position of atoms and the bonds between them
Skeletal formula
Drawn by using lines to represent alkyl groups
Homologous series
A series of organic compounds with the same functional group but with each successive member
differing by CH2
Functional groups
The part of the compound largely responsible for its chemical properties
Alkyl group
C(n)H(2n+1)
Aliphatic
Carbon atoms are joined to each other in un-branched or branched chains
Alicyclic
Carbon atoms are joined to each other in a ring
Aromatic
Some or all of the carbon atoms are found in a benzene ring
Saturated
The presence of only carbon-carbon single bonds
Un-saturated
The presence of carbon-carbon double or multiple bonds
Structural formula
Compounds with the same molecular formula but with different structures
Homolytic fission
Each bonding atom receives one electron from the
bonded pair, forming two radicals
Heterolytic fission
One bonding atom receives both electrons from the bonded pair
Radical
A species with an unpaired electron, it is represented with a dot
What are curly arrows used for
The movement of a pair of electrons showing either heterolytic fission or the formation of a covalent bond
Alkanes
Saturated hydrocarbons, containing only c-c and c-h bonds
What type of bonds do Alkanes have
Each carbon atom in an alkane is joined to 4 other atoms by a single covalent bond. This covalent bond is a sigma bond.
How are sigma bonds formed
The overlap of two orbitals, one from each bonding atom. The sigma bond has free rotation
The shape of alkanes
Each carbon atom is surrounded by 4 electron pairs, repulsion between these electron pairs results in a tetrahedral arrangement around each carbon atom. The bond angle is 109.5 degrees
The effect of chain length on the boiling point of alkanes
As chain length increases, the molecules have a larger surface area so there is more surface contact between which London forces can form. The London forces between the molecules will be greater and more energy is required to overcome these forces so they will have higher boiling points. More electrons stronger London forces
The effect of branching on the boiling point of alkanes
As branching increases there is less surface area of contact between the molecules where London forces can form. The London forces between the molecules will be weaker and less energy will be required to overcome these forces so boiling point will be lower
Why are alkanes un-reactive?
The C-H and C-C sigma bonds are strong. The C-C bonds are non-polar. The electronegativity of carbon and hydrogen is so similar that the C-H bond can be considered non-polar
Complete combustion of alkanes
They react with oxygen to produce CO2 and water
Incomplete combustion of alkanes
Happens when there is not enough oxygen for complete combustion. Instead of carbon dioxide, carbon monoxide or just carbon is formed. Carbon monoxide is a toxic gas which combines with haemoglobin to form carboxyhaemoglobin and stops the haemoglobin from transporting oxygen around the body. They don’t get enough oxygen and death results
Reaction of alkanes with halogens
Occurs in the presence of UV light. It is a substitution reaction with one Bromine atom combining with the alkane and the hydrogen atom it has replaced combining with the other Bromine atom.
What type of mechanism is Bromine reacting with an alkane
Radical substitution
Initiation step of Bromine reacting with an alkane
The covalent bond in the Bromine molecule is broken by homolytic fission. Each Bromine atom takes one electron from the pair forming two highly reactive Bromine radicals, the energy for this is provided by UV radiation. Br-Br –> Br* +Br*
Propagation step of Bromine reacting with an alkane
CH4 + Br* –> CH3+ HBr
CH3 + Br2 –> CH3Br +Br*
This is a chain reaction meaning the 2 steps can continue to cycle through and cause each other. The propagation step can only stop when two radicals collide with each other.
Termination step of Bromine reacting with an alkane
Br* + Br* –> Br2
CH3* + CH3* –> C2H6
CH3* + Br* –> CH3Br
In the termination stage two radicals collide, forming a molecule with all electrons paired. When the radicals are removed from the mixture the reaction stops.
Limitations of radical substitution in organic synthesis;
Further substitutions
Another Bromine radical can collide with CH3Br, substituting another hydrogen atom to form CH2Br2. Further substitution can continue until all hydrogen atoms have been substituted. The result is a mixture of different bromomethane molecules with differing numbers of Bromine atoms on it.
Limitations of radical substitution in organic synthesis;
Substitution at different points in the carbon chain
If the Carbon chain is longer then ethane then you will get a mixture of alkanes with bromine substituted at different positions in the carbon chain.
Alkenes
They are unsaturated hydrocarbons containing a C=C bond
The type of bonds in Alkenes
In each carbon of the double bond 3 of the 4 electrons are used in sigma bonding. The one electron not involved in sigma bonding is in the P orbital. A pie bond is formed by the sideways overlap of two p-orbitals, one from each carbon atom in the double bond. The pie bond is concentrated above and below the plane of the carbon atoms.
Significance of Pie bond in alkenes
The pie bond locks the two carbon atoms in position and prevents them from rotating around the double bond.
Shape of Alkenes
The shape around each carbon in a double bond is trigonal planar. There are three bonded pairs of electron around each of the Carbon atoms. These three regions repel each other as far as possible so the bond angle around each Carbon atom is 120 degrees.
Stereoisomerism
Compounds with the same structural formula but with a different arrangement in space
E/Z isomerism
This is a type of stereoisomerism, it is around the double bond and is because rotation around the double bond is restricted. Different groups must also be attached to each carbon atom of the double bond.
E isomerism
When the two groups are on different sides from each other from each other. For example one is up an the other down
Z isomerism
When the two groups are on the same side as each other
Cis-trans isomerism
A special type of E/Z isomerism in which two of the substituent groups attached to each carbon in the C=C group are the same.
Cis isomerism
Same as Z isomer, so when its on the same side
Trans isomerism
Same as the E isomer, so when the groups are on different sides.
CIP rule about isomers
In stereoisomers, the atoms with the highest atomic mass are given priority in naming. So if the groups with the highest atomic mass are on the same side of the double bond then it is a Z isomer.
Reactivity of Alkenes
Alkenes are more reactive then alkanes due to the presence of a pie bond which creates a region of electron density. Being on the outside of the double bond, the pie electrons are more exposed then the electrons in the sigma bond. A pie bond readily breaks and alkenes undergo addition reactions relatively easily.
Hydrogenation of alkenes
An alkene reacts with hydrogen in the presence of a nickel catalyst to form an alkane. This is an addition reaction
Halogenation of alkenes
Alkenes undergo an addition reaction with either chlorine or bromine at room temperature. The double bond is broken and both halogen atoms join on one carbon each to form a dihaloalkane.
Test for a double C=C bond
When bromine is added to an alkene, the colour goes from orange to colourless as bromine adds across the double bond. If the same test is carried out with an alkane or another saturated hydrocarbon no colour change will occur.
Addition reaction of alkenes with hydrogen halides
Alkenes react with gaseous hydrogen halides at room temperature to form haloalkanes. The double bond is broken and a hydrogen will add to one carbon and a halide to another. There may be a mixture of products with the halide joining at different places on the Carbon chain.
Hydration reaction of alkenes
Alcohols are formed when alkenes react with steam in the presence of a phosphoric acid catalyst. The double bond is broken and hydrogen bonds with one carbon atom and an OH group to another.
Electrophile
An electron pair acceptor
Markownikoff’s rule
The major product of a reaction with an alkene and a hydrogen halide will be when the bromine attaches to the carbon with the most carbons attached. So not at the end of a carbon chain.
Polymers
Extremely large molecules made from thousands of repeat units called monomers.
Monomer unit
W X
n C=C
Y Z
Repeat Unit
[ W X ]
——C–C———
[ Y Z ] n
Combustion of polymers for energy production
Polymers have a high stored energy value. Waste polymers can be burnt to produce heat which generates steam which drives a turbine to produce electricity. This is useful as some polymers can not be recycled
Using polymers as feedstock
The polymers can be used for feedstock for the production of plastics and other organic chemicals. It is able to handle unsorted and unwashed polymers
Why are polymers bad for the environment
As they are so un-reactive they are often not biodegradable.
Uses of poly(ethene)
Supermarket bags, shampoo bottles and children’s toys
Uses of poly(chloroethene)
Pipes, insulation and films and sheeting
Uses of poly(propene)
Making children’s toys, packing crates and guttering
Uses of poly(phenylethene)
Packing material, food trays and cups.
Uses of poly(tetrafluoroethene)
Coating for non-stick pans and cable insulation
Why are polymers so useful
They are readily available, cheap to produce and light
PVC recycling
Recycling PVC is dangerous due to the high chlorine content. When burnt the gases are treated to remove the HCl.
Biodegradable polymers
They can be broken down by microorganisms. The polymers are usually made from starch or cellulose. They leave no visible or toxic residue. Plant based polymers
Photodegradable polymers
These polymers contain bonds that are weakened by absorbing light, which starts their degradations
Margarine
Vegetable oil contains unsaturated hydrocarbon chains in the cis orientation. Hydrogen gas is bubbled through in the presence of a nickel catalyst. Many of the unsaturated double bonds are hydrogenated to form saturated carbon chains. The hydrogenated products have a higher melting point and are more solid.
