C11 & 12 - Organic Chemistry and Alkanes Flashcards
What’s a functional group?
An atom or group of atoms responsible for the typical chemical reactions of a molecule.
They determine the pattern of reactivity of a homologous series whereas the carbon chain length determines physical properties e.g. Melting and boiling points.
What are the three groups used for the classification of hydrocarbons?
Aliphatic
Alicyclic
Aromatic
What are aliphatic, alicyclic and aromatic hydrocarbons?
Aliphatic - Hydrocarbons with carbon atoms joined in straight un/branched chains.
Alicyclic - Hydrocarbons with carbon atoms joined in cyclic ring structures.
Aromatic - Hydrocarbons with some or all carbon atoms found in a benzene ring.
How are aliphatic alkanes named?
1) Identify the longest continuous chain of carbon atoms
2) Identify any side chains attached, known as alkyl groups. The alkyl group’s prefix is added to the parent chain’s name.
3) Add numbers before any alkyl groups to show the position of the group on the parent chain.
E.g. 4-ethyl-2,3-dimethylhexane
(For an alicyclic hydrocarbon, cyclo- is put at the start of the stem)
What is the functional group and suffix of an: alkene,
alcohol,
halo alkane and
aldehyde?
Alkene: c=c / -ene
Alcohol: -OH / -ol
Halo alkane: -Cl, -Br etc
Aldehyde: -CHO / -al
What is the functional group and suffix of an: Ketone Carboxylic acid Ester Acyl chloride Amine Nitrile
Ketone: -C(CO)C- / -one
Carboxylic acid: -COOH / -oic acid
Ester: -COO / -oate
Acyl chloride: -COCl / -oly chloride
Amine: -NH2 / -amine
Nitrile: -CN / -nitrile
What is the general formula for an alcohol, carboxylic acid and ketone?
Alcohol: Cn H2n+1 OH
Carboxylic acid: Cn H2n O2
Ketone: Cn H2n O
What does a displayed formula show?
Relative positioning of atoms and the bonds between them e.g.
H H-C- C= O H O-H
What does a structural formula show?
Minimal detail needed to show the arrangement of the atoms in a molecule e.g.
CH3CH2CH2CH3
What are structural isomers?
Compounds with the same molecule formula but different structural formulae.
If they have the same functional group, the functional group can be at different positions along the carbon chain.
What is a homologous series?
A group of molecules with the same functional group but a different number of -CH2 groups.
(The simplest is the alkanes)
What is structural isomers?
Compounds with wig the same molecular formula but different structural formulae.
What is a covalent bond?
The electrostatic attraction between a shared pair of electrons and the nuclei of atoms.
What are the two types of bond fission?
Homolytic fission
Heterolytic fission
What is homolytic fission?
A form of bond fission where, once the covalent bond breaks, each of the bonded atoms take ONE of the shared electrons from the bond.
Each atom now has a single unpaid electron.
What type of particles are formed by homolytic fission?
Radicals
What is a radical?
An atom or group of atoms with an unpaired electron
What is heterolytic fission?
A form of fission where, once the covalent bond breaks, one of the bonding atoms takes BOTH of the electrons.
The atom that takes both electrons becomes a negative ion and the other is a positive ion.
What do curly arrows show?
The movement of electron pairs when bonds are being broken or made.
What are the three types of reaction?
Addition
Substitution
Elimination
What occurs during an addition reaction?
Two reactants join to form one product.
What occurs during a substitution reaction?
An atom or group of atoms is replaced by a different atom or group of atoms.
What occurs during an elimination reaction?
One reactant molecule forms two products. It involves the removal of a small molecule from a larger one.
What’s the general formula of an alkane?
CnH2n+2
A saturated hydrocarbon
What type of bonding is present in alkanes?
Single covalent bonds joining the C’s and H’s
These types of covalent bonds are called sigma bonds.
What is a sigma bond?
Is a type of covalent bond as the result of two overlapping orbitals, one from each bonding atom.
The sigma bond has two electrons shared between binding atoms and is on a line directly between bonding atoms.
What is the shape of alkanes?
Each C atom is surrounded by 4 electron pairs in four sigma bonds.
Repulsion results in a 3D tetrahedral shape around each C atom with a bond angle approx 109.5°.
Why does the boiling point of alkanes increase as their size increases?
The greater the number of atoms, the stronger the London forces between the atoms therefore more energy is required to overcome the bonds.
How does the length of the alkane chain affect it’s boiling point?
London forces act between molecules that are in close surface contact.
As chain length increases, the molecules have a larger surface area so more surface contact is possible between molecules.
The London forces between molecules will be greater so more energy is needed to overcome the forces.
How does branching affect the boiling point of alkanes?
Branched isomers have lower boiling points.
This is because there are fewer surface points of contact between molecules of the branched alkanes so there are fewer London forces.
The branches also get in the way and prevent the branched molecules from getting as close together as straight chain molecules, also decreasing intermolecular forces.
What is the equation for the complete combustion of an alkane? (Fuel)
Alkane + oxygen -> carbon dioxide + water
What is formed during incomplete combustion?
Carbon monoxide - toxic
What causes alkanes to react with halogens?
Ultraviolet radiation in sunlight provides the initial energy for the reaction to take place.
What is radical substitution?
A reaction mechanism showing the movement of electrons in 3 stages.
What are the 3 steps of radical substitution?
Initiation
Propagation
Termination
What occurs during the 3 stages of radical substitution?
- Initiation: the reaction starts when the covalent bond in a halogen molecule is broken by homolytic fission.
This forms two reactive halogen radicals. - Propagation: the reaction propagates through a two step propagation chain reaction.
In the first step, a halogen radical reacts with a C-H bond forming an ester radical and a molecule of hydrogen + the halogen.
In the second step, each ester radical radical reacts with another halogen molecule, forming the organic product and another halogen radical.
The two steps continue to cycle in a chain reaction. - Termination: two radicals (one of each kind) collide, forming a molecule with all electrons paired. The are various possibilities.
When two radicals collide and react, both radicals are removed, stopping the reaction.
What happens in the initiation stage of radical substitution?
The reaction starts when the covalent bond in a halogen molecule is broken by homolytic fission.
This forms two reactive halogen radicals.
What happens in the propagation stage of radical substitution?
The reaction propagates through a two step propagation chain reaction.
In the first step, a halogen radical reacts with a C-H bond forming an ester radical and a molecule of hydrogen + the halogen.
In the second step, each ester radical radical reacts with another halogen molecule, forming the organic product and another halogen radical.
The two steps continue to cycle in a chain reaction.
What happens in the termination stage of radical substitution?
Two radicals (one of each kind) collide, forming a molecule with all electrons paired. The are various possibilities. When two radicals collide and react, both radicals are removed, stopping the reaction.
Why aren’t alkanes very reactive?
The C-C and C-H sigma bonds are strong
C-C bonds are non-polar
The electro negativity of carbon and hydrogen is so similar that the C-H bond can be considered to be non-polar
What is the balanced equation for the combustion of ethane?
C2H6 + 3.5 H2O -> 2CO2 + 3H2O
