, ALKANES
Carbon can form a vast number of compounds because…
…C atoms can form 4 covalent bonds to other atoms.
…C can form single, double or triple bonds.
…C atoms can bond to other C atoms to form long chains.
A hydrocarbon is a compound containing carbon and hydrogen only.
Specialist terms used to describe certain types of organic molecules:
- aliphatic = a compound containing carbon and hydrogen joined together in straight chains, branched chains or non-
aromatic rings.
- alicyclic = an aliphatic compound arranged in non-aromatic rings with or without side chains.
- aromatic = a compound containing a benzene ring.
- saturated = a compound containing single carbon-carbon bonds only.
- unsaturated = a compound containing multiple carbon-carbon bonds, including C=C, C≡C & aromatic rings.
Types of formulae:
- empirical = the simplest whole number ratio of atoms of each element present in a compound.
- skeletal = a simplified organic formula with the hydrogen atoms removed from alkyl chains, leaving just a carbon
skeleton and associated functional groups.
- general = the simplest algebraic formula of a member of a homologous series.
- molecular = the actual number of atoms of each element in a molecule.
- structural = the minimal detail that shows the arrangement of atoms in a molecule.
- displayed = the relative positioning of all the atoms in a molecule, including the bonds between them.
A functional group is a group of atoms responsible for the characteristic reactions of a compound.
Structural isomers are compounds with the same molecular formula but different structural formulae.
Structural isomerism can occur in three ways:
- chain isomers = the hydrocarbon chain may be unbranched or branched.
- position isomers = the hydrocarbon chain stays the same in these isomers, but the functional group is found in
different positions along the chain.
- functional group isomers = the functional group is different.
e.g. the following are structural isomers with the molecular formula C4H8:
A homologous series is a series of organic compounds with the same functional group but with each successive
member differing by CH2.
Alkanes are the simplest homologous series of saturated, aliphatic hydrocarbons.
The general formula of the alkanes is CnH2n+2.
Cycloalkanes are another homologous series of saturated, alicyclic hydrocarbons.
The general formula of the cycloalkanes is CnH2n.
The shape around each carbon atom in alkanes is tetrahedral – each carbon atom is surrounded by four bonded
pairs of electrons; its four electron pairs repel one another as far apart as possible in three-dimensional space.
The H-C-H bond angle in alkanes is 109.5˚C.
,The effect of chain length on the boiling points of alkanes:
- trend: as the number of carbon atoms increases, so does the boiling point of the alkane.
- reason: with more atoms, there are more electrons, meaning the strength of the London forces increases, and a
greater quantity of energy is required to separate the molecules for them to move apart and become gaseous.
The effect of branching on the boiling points of alkanes:
- trend: as the number of branches increases, the boiling point decreases.
- reason: with more branching, there is a smaller contact area between the molecules, meaning the strength of the
London forces decreases, and a smaller quantity of energy is required to separate the molecules for them to move
apart and become gaseous.
Alkanes are relatively unreactive because…
…they only contain C-C and C-H bonds, which are strong, and therefore require large quantities of energy to break.
…it is difficult for alkanes to attract attacking species due to the fact that C-C bonds are non-polar, and the
electronegativities of C and H are similar enough that the C-H bond is considered non-polar.
In the complete combustion of an alkane, the alkane is burned cleanly in a plentiful supply of oxygen to produce
carbon dioxide and water.
A blue flame is seen during complete combustion.
The general equation for the complete combustion of an alkane is:
CxHy + (x + ¼ y)O2 → xCO2 + ½ yH2O
In the incomplete combustion of an alkane, the alkane is burned in a limited supply of oxygen to produce carbon
monoxide and/or carbon (soot), as well as water.
A yellow flame is seen during incomplete combustion.
Incomplete combustion can be dangerous because carbon monoxide is toxic and causes death by preventing
haemoglobin from carrying oxygen to respiring tissues. It is also colourless and odourless, so is difficult to detect.
Alkanes undergo FREE RADICAL SUBSTITUTION reactions with halogens to form haloalkanes:
A RADICAL is a species with an unpaired electron.
A SUBSITUTION REACTION is a reaction in which an atom or group of atoms is replaced with a different atom or
group of atoms.
Conditions:
- UV light: provides the activation energy required for the initiation stage of free radical substitution by breaking the
X-X bond.
Type of bond fission:
- Homolytic: each bonding atom receives one electron from the bonded pair, forming two radicals.
Free radical substitution has limitations
which make it difficult to synthesise only one
desired compound:
- further substitution can occur.
- substitution at different positions on the C
chain can occur.
- the C chain may change in length (e.g.
double).
, ALKENES
Alkenes are unsaturated hydrocarbons containing a C=C bond.
The general formula of the alkenes is CnH2n.
Single bonds comprise of a single σ-bond.
Double bonds comprise of a σ-bond and a π-bond.
Sigma (σ) bonds…
…are the result of the end-on overlap of atomic orbitals.
…are free to rotate.
…have their electron density directly between the bonding atoms.
Pi (π) bonds…
…are the result of the sideways overlap of two p-atomic-orbitals.
…cannot rotate as this would break the overlap of the p-orbitals.
…have their electron density above and below the plane of the bonding atoms.
…are slightly weaker than σ-bonds due to the position of their electron density.
The shape around the carbon atoms in a C=C bond is trigonal planar – it has four bonded pairs of electrons, two of
which form a double bond - three bonding regions overall - around its central carbon atom; its three bonding regions
repel one another as far apart as possible in three-dimensional space.
The H-C=C bond angle is 120˚C.
Stereoisomers are molecules that have the same structural formula but a different arrangement of atoms in three-
dimensional space.
E/Z isomerism is an example of stereoisomerism.
E/Z isomers occurs when…
…there is restricted rotation around a C=C double bond.
…each of the carbon atoms of the C=C double bond has two different groups attached.
The CIP (Cahn-Ingold-Prelog) rules are used to determine which isomer is the E isomer and which is the Z isomer:
- the isomer that has the two groups of higher priority on opposite sides of the double bond is the E isomer, and the
isomer that has the two groups of higher priority on the same side of the double bond is the Z isomer.
- the higher the atomic number of an atom attached to a carbon atom of the double bond, the higher its priority.
- if the two atoms attached to a carbon atom of the double bond are the same, the group which has the higher
atomic number at the first point of difference is given the higher priority.
CIP nomenclature: (E)- or (Z)- is added before the name of the stereoisomer depending on whether it is an E or Z
isomer respectively.
Cis/trans isomerism is a special case of E/Z isomerism in which two of the substituent groups attached to each
carbon atom of the C=C double bond are the same:
- cis- => the identical groups are on the same side of the double bond.
- trans- => the identical groups are on opposite sides of the double bond.
Alkenes undergo ELECTROPHILIC ADDITION with hydrogen to form alkanes:
An ELECTROPHILE is an electron pair acceptor.
An ADDITION REACTION is a reaction in which a reactant is added to an unsaturated molecule to form one saturated
molecule.
Conditions:
- Nickel catalyst
- 150˚C
Type of bond fission:
- heterolytic: both electrons from the bonded pair go to the same bonding atom, forming a cation and an anion.
Carbon can form a vast number of compounds because…
…C atoms can form 4 covalent bonds to other atoms.
…C can form single, double or triple bonds.
…C atoms can bond to other C atoms to form long chains.
A hydrocarbon is a compound containing carbon and hydrogen only.
Specialist terms used to describe certain types of organic molecules:
- aliphatic = a compound containing carbon and hydrogen joined together in straight chains, branched chains or non-
aromatic rings.
- alicyclic = an aliphatic compound arranged in non-aromatic rings with or without side chains.
- aromatic = a compound containing a benzene ring.
- saturated = a compound containing single carbon-carbon bonds only.
- unsaturated = a compound containing multiple carbon-carbon bonds, including C=C, C≡C & aromatic rings.
Types of formulae:
- empirical = the simplest whole number ratio of atoms of each element present in a compound.
- skeletal = a simplified organic formula with the hydrogen atoms removed from alkyl chains, leaving just a carbon
skeleton and associated functional groups.
- general = the simplest algebraic formula of a member of a homologous series.
- molecular = the actual number of atoms of each element in a molecule.
- structural = the minimal detail that shows the arrangement of atoms in a molecule.
- displayed = the relative positioning of all the atoms in a molecule, including the bonds between them.
A functional group is a group of atoms responsible for the characteristic reactions of a compound.
Structural isomers are compounds with the same molecular formula but different structural formulae.
Structural isomerism can occur in three ways:
- chain isomers = the hydrocarbon chain may be unbranched or branched.
- position isomers = the hydrocarbon chain stays the same in these isomers, but the functional group is found in
different positions along the chain.
- functional group isomers = the functional group is different.
e.g. the following are structural isomers with the molecular formula C4H8:
A homologous series is a series of organic compounds with the same functional group but with each successive
member differing by CH2.
Alkanes are the simplest homologous series of saturated, aliphatic hydrocarbons.
The general formula of the alkanes is CnH2n+2.
Cycloalkanes are another homologous series of saturated, alicyclic hydrocarbons.
The general formula of the cycloalkanes is CnH2n.
The shape around each carbon atom in alkanes is tetrahedral – each carbon atom is surrounded by four bonded
pairs of electrons; its four electron pairs repel one another as far apart as possible in three-dimensional space.
The H-C-H bond angle in alkanes is 109.5˚C.
,The effect of chain length on the boiling points of alkanes:
- trend: as the number of carbon atoms increases, so does the boiling point of the alkane.
- reason: with more atoms, there are more electrons, meaning the strength of the London forces increases, and a
greater quantity of energy is required to separate the molecules for them to move apart and become gaseous.
The effect of branching on the boiling points of alkanes:
- trend: as the number of branches increases, the boiling point decreases.
- reason: with more branching, there is a smaller contact area between the molecules, meaning the strength of the
London forces decreases, and a smaller quantity of energy is required to separate the molecules for them to move
apart and become gaseous.
Alkanes are relatively unreactive because…
…they only contain C-C and C-H bonds, which are strong, and therefore require large quantities of energy to break.
…it is difficult for alkanes to attract attacking species due to the fact that C-C bonds are non-polar, and the
electronegativities of C and H are similar enough that the C-H bond is considered non-polar.
In the complete combustion of an alkane, the alkane is burned cleanly in a plentiful supply of oxygen to produce
carbon dioxide and water.
A blue flame is seen during complete combustion.
The general equation for the complete combustion of an alkane is:
CxHy + (x + ¼ y)O2 → xCO2 + ½ yH2O
In the incomplete combustion of an alkane, the alkane is burned in a limited supply of oxygen to produce carbon
monoxide and/or carbon (soot), as well as water.
A yellow flame is seen during incomplete combustion.
Incomplete combustion can be dangerous because carbon monoxide is toxic and causes death by preventing
haemoglobin from carrying oxygen to respiring tissues. It is also colourless and odourless, so is difficult to detect.
Alkanes undergo FREE RADICAL SUBSTITUTION reactions with halogens to form haloalkanes:
A RADICAL is a species with an unpaired electron.
A SUBSITUTION REACTION is a reaction in which an atom or group of atoms is replaced with a different atom or
group of atoms.
Conditions:
- UV light: provides the activation energy required for the initiation stage of free radical substitution by breaking the
X-X bond.
Type of bond fission:
- Homolytic: each bonding atom receives one electron from the bonded pair, forming two radicals.
Free radical substitution has limitations
which make it difficult to synthesise only one
desired compound:
- further substitution can occur.
- substitution at different positions on the C
chain can occur.
- the C chain may change in length (e.g.
double).
, ALKENES
Alkenes are unsaturated hydrocarbons containing a C=C bond.
The general formula of the alkenes is CnH2n.
Single bonds comprise of a single σ-bond.
Double bonds comprise of a σ-bond and a π-bond.
Sigma (σ) bonds…
…are the result of the end-on overlap of atomic orbitals.
…are free to rotate.
…have their electron density directly between the bonding atoms.
Pi (π) bonds…
…are the result of the sideways overlap of two p-atomic-orbitals.
…cannot rotate as this would break the overlap of the p-orbitals.
…have their electron density above and below the plane of the bonding atoms.
…are slightly weaker than σ-bonds due to the position of their electron density.
The shape around the carbon atoms in a C=C bond is trigonal planar – it has four bonded pairs of electrons, two of
which form a double bond - three bonding regions overall - around its central carbon atom; its three bonding regions
repel one another as far apart as possible in three-dimensional space.
The H-C=C bond angle is 120˚C.
Stereoisomers are molecules that have the same structural formula but a different arrangement of atoms in three-
dimensional space.
E/Z isomerism is an example of stereoisomerism.
E/Z isomers occurs when…
…there is restricted rotation around a C=C double bond.
…each of the carbon atoms of the C=C double bond has two different groups attached.
The CIP (Cahn-Ingold-Prelog) rules are used to determine which isomer is the E isomer and which is the Z isomer:
- the isomer that has the two groups of higher priority on opposite sides of the double bond is the E isomer, and the
isomer that has the two groups of higher priority on the same side of the double bond is the Z isomer.
- the higher the atomic number of an atom attached to a carbon atom of the double bond, the higher its priority.
- if the two atoms attached to a carbon atom of the double bond are the same, the group which has the higher
atomic number at the first point of difference is given the higher priority.
CIP nomenclature: (E)- or (Z)- is added before the name of the stereoisomer depending on whether it is an E or Z
isomer respectively.
Cis/trans isomerism is a special case of E/Z isomerism in which two of the substituent groups attached to each
carbon atom of the C=C double bond are the same:
- cis- => the identical groups are on the same side of the double bond.
- trans- => the identical groups are on opposite sides of the double bond.
Alkenes undergo ELECTROPHILIC ADDITION with hydrogen to form alkanes:
An ELECTROPHILE is an electron pair acceptor.
An ADDITION REACTION is a reaction in which a reactant is added to an unsaturated molecule to form one saturated
molecule.
Conditions:
- Nickel catalyst
- 150˚C
Type of bond fission:
- heterolytic: both electrons from the bonded pair go to the same bonding atom, forming a cation and an anion.
