Basic concepts and alkanes Flashcards
What is a general formula?
An algebraic formula that can describe any member of a family of compounds.
What is the general formula of an alcohol?
CnH2n+1OH
What is a homologous series?
A family of compounds that have the same functional group and general formula, but each successive member differs by CH2.
What is the general formula of an alkane?
CnH2n+2
What is molecular formula?
It gives the actual number of atoms of each element in a molecule.
What is empirical formula?
The simplest whole number ratio of atoms of each element in a compound.
Divide the molecular formula by the smallest number of atoms for a given element in the molecule.
What are aromatic compounds?
They contain a benzene ring.
What are aliphatic compounds?
They contain hydrogen and carbon joined together in straight chains, branched chains or non-aromatic rings.
What are alicyclic compounds?
When an aliphatic compound contains a ring that is not benzene.
What is an alkyl group?
In compounds with a branched carbon skeleton, it is any hydrocarbon branched chain.
The general formula is CnH2n+1.
What are saturated compounds?
They contain only carbon-carbon single bonds.
What are unsaturated compounds?
They contain multiple carbon-carbon bonds.
What are structural isomers?
They have the same molecular formula, but the structural formula is different.
What are chain isomers?
Molecules that have different arrangements of the carbon skeleton.
They have similar chemical properties, but different physical properties, due to the change in shape of the molecule.
What are positional isomers?
The skeleton and functional group can be the same, but the functional group could be attached to a different carbon atom.
They have different physical properties, and chemical can also be different.
What are cycloalkanes?
They have a ring of carbon atoms with 2 hydrogens attached to each carbon.
They have 2 less hydrogens than other alkanes.
What are IUPAC rules?
Count the carbon atoms in the longest continuous chain to get the stem.
Number the chain so the main functional group has the lowest number. Or pick the one with the most side chains.
Put the side chains in alphabetical order after the number it is attached to, at the start.
Use di, tri or tetra if there’s multiple identical things.
What is the bonding in alkanes?
Atomic orbitals on carbon and hydrogen atoms are positioned so that they overlap to form a new, shared orbital lying directly between the bonded atoms.
This type of covalent bond is a sigma bond.
What is the strength of sigma bonds?
The high electron density between the nuclei means there is strong electrostatic attraction between the nuclei and the shared pair of electrons.
So they have high bond enthalpy.
What is the rotation of sigma bonds?
The bonded atoms are free to rotate around the sigma bond, because the atomic orbitals between them will always overlap.
What is a sigma bond?
Overlap of orbitals directly between bonding atoms.
What is the reactivity of alkanes?
Low reactivity because sigma bonds are very strong and so difficult to break.
They are also non-polar - won’t attract positively or negatively charged particles to react with them.
What are the boiling points of alkanes?
It depends on the size and shape.
Small alkanes, e.g. methane, are gases at room temperature and pressure.
Larger alkanes are liquid.
How does the intermolecular forces of alkanes affect boiling points?
The longer the carbon chain, the more induced dipole dipole interactions.
This is because of larger surface contact, and so more electrons to interact.
So it takes more energy to overcome the interactions and boiling point increases.
How does branching of alkanes affect boiling point?
Branched alkanes have a lower boiling point because it can’t pack closely together and so has less surface contact, so induced dipole dipole interactions are reduced.
What is the complete combustion of alkanes?
Burning alkanes with enough oxygen produces carbon dioxide and water.
Which alkanes burn best?
Complete combustion happens to gases, so liquids must be vaporised first.
Smaller alkanes turn into gases easier (more volatile) so will burn more easily.
Which alkanes produce more energy in combustion?
Some energy is used to break the covalent bond but more energy is produced in forming the new product bonds.
So larger alkanes release more energy per mole than smaller, as more CO2 and H2O are formed.
They make excellent fuels.
What is incomplete combustion of alkanes?
When there is limited oxygen, so carbon monoxide and water are formed.
Why is carbon monoxide bad?
It is poisonous, and better at binding to haemoglobin than oxygen is.
So less oxygen can reach the cells and they suffer oxygen deprivation - fatigue, headaches, nausea.
What is bond fission?
Breaking a covalent bond, a shared pair of electrons between two atoms.
What is heterolytic fission?
The bonds break unevenly with one of the atoms recieving both electrons from the bonded pair.
A cation and an anion are formed.
What is homolytic fission?
The bond breaks evenly and each atom recieves one electron from the bonded pair.
Two electrically uncharged radicals are formed.
Because of the unpaired electron, radicals are very reactive.
What do curly arrows show?
It shows the movement of a pair of electrons during a reaction.
How are haloalkanes formed?
When halogens react with alkanes in photochemical reactions - they’re started with light.
A hydrogen atom is substituted by a halogen such as chlorine or bromine in radical substitution
What is the synthesis of chloromethane?
CH4 + Cl2 -UV-> CH3Cl + HCl
Initiation: Cl2 -UV-> 2Cl*
Propagation: Cl* + CH4 –> CH3* + HCl
CH3* + Cl2 –> CH3Cl + Cl*
Termination: Cl* + Cl* –> Cl2
CH3* + Cl* –> CH3Cl
CH3* + CH3* –> C2H6
What are the steps of radical substitution?
Initiation: UV light produces radicals.
Propagation: Radicals are used up and created in a chain reaction. Cl* attacks methane. The new methyl radical CH3* attacks Cl2. The new Cl* attacks CH4. This continues until all the Cl2 or CH4 are used up.
Termination: Two radicals join up to form a stable molecule and terminate the reaction.
What are the limitations of radical substitution?
A mixture of products is made, so the desired product must be separated from the unwanted by-products.
The chance of this happening is reduced by using an excess of alkane.
It can also take place at any point on the carbon chain, so a mixture of isomers is formed.
