Briefly describe covalent bonding
Covalent bonds form most of the bonds in organic compounds. When forming covalent bonds, atoms tend to attain a noble gas configuration
State the three things to consider when looking at how covalent bonds form
- The total number of electrons in an atom
- The number of electrons in the outer shell
- The valency (ie the number of bonds)
A homologous series containing at least one carbon-carbon triple bond
Compounds in which one carbon atom is double bonded to two other carbon atoms
Define the node/nodal plane
An area of low electron density
Describe how to count electrons for any atom in a molecule
The purpose is to determine how many electrons the atom owns in the molecule compared to how many it owns as an isolated atom. It owns one electron per bond and two per lone pair
Describe s orbitals
S orbitals are spherical
Describe p orbitals
p orbitals are dumbbell shaped with a node in the middle. There are three p orbitals, labelled x y and z and they are superimposed at 90 degrees to each other.
Describe hybrid orbitals
When bonds are formed, various combinations of s and p orbitals give hybrid orbitals, which define the shapes of molecules
Describe sp3 hybridisation
One s and three p orbitals mix to form four identical sp3 hybrid orbitals. Each orbital is dumbbell shaped with one lobe smaller than the other, and the four have corners forming a tetrahedron, and are 109.5 degrees apart from each other
Describe the bonding in methane
Each sp3 hybrid orbital forms a sigma bond (a region of high electron density) with a hydrogen atom.
Describe sp2 hybridisation
One s and two p orbitals mix to form three identical sp2 hybrid orbitals. Each orbital is dumbbell shaped with one lobe smaller than the other, and the three have corners forming a linear equilateral triangle (trigonal orientation), and are 120 degrees apart from each other. The remaining p orbital is perpendicular to this plane.
State the multiplier given by 'pico'
Define a double bond and state its bond length in carbon
A double bond is formed of one sigma bond and one pi bond. A carbon carbon double bond is approximately 134pm
Describe sp hybridisation
One s and one p orbital mix to form two identical sp hybrid orbitals. Each orbital is dumbbell shaped with one lobe smaller than the other, and point at 180 degrees to each other. The other two 2p orbitals are perpendicular to each other and are in line with the sp orbitals.
Define a triple bond and state its bond length in carbon
A triple bond is formed of one sigma bond and two pi bonds between the same two atoms. A carbon carbon triple bond is approximately 120pm.
Describe how to identify sp hybridisation by looking at the number of pi and sigma bonds
If a carbon atom has 2 pi bonds and 2 sigma bonds, it is sp hybridised
Describe how to identify sp2 hybridisation by looking at the number of pi and sigma bonds
If a carbon atom has 1 pi bond and 3 sigma bonds, it is sp2 hybridised
Describe how to identify sp3 hybridisation by looking at the number of pi and sigma bonds
If a carbon atom has 0 pi bonds and 4 sigma bonds, it is sp3 hybridised.
State which out of pi and sigma bonds are weaker
Pi bonds are weaker than sigma bonds.
Define the electronegativity of an element
Electronegativity is a measure of an element's tendency to attract the electron density in a covalent bond
Describe what happens in a bond if there is a difference in electronegativity between the atoms
Bonds are polarised, as one atom will be pulling the bonding electrons more forcefully than the other. This forms a dipole, and can be shown using an arrow (->-) or with partial charges. The bigger the difference in electronegativity, the bigger the partial charges
Describe how positive charges are affected in molecules with more than two atoms
The main influence of the electronegative atom is felt by the atom(s) directly bonded to it, but its effect is felt further along the chain, although the 'inductive effect' falls off very quickly. It can be represented by drawing more delta signs further along the chain
Describe the inductive effect, using H-CH2-Cl as an example
The inductive effect refers to the electron withdrawing effect (due to a difference in electronegativity) through the sigma bonds of a molecule. In chloromethane, the chlorine atom would be referred to as an inductively electron-withdrawing substituent
List substituents in order of strongest electron withdrawing ability to strongest electron donating ability
- Nitro groups
- Alcohols & ethers
Explain why alcohols and ethers are lower in the list of electron withdrawing ability than may have been expected
The effect from the oxygen atom is ‘diluted’ as it is working in two directions, not one.
State what is meant by drawing a solid wedge
The substituent at the end of a solid wedge is pointing towards you, out of the plane
State what is meant by drawing a dashed wedge
The substituent at the end of a dashed wedge is pointing away from out, into the plane
Enantiomers are a pair of compounds which are non-superimposable mirror images of one another. They are a form of optical isomers
Describe a chiral molecule
A chiral molecule is one that is not superimposable on its mirror image
Describe a chiral centre
A chiral centre is most commonly a carbon atom with four different groups attached to it. It can also be called a stereogenic centre
Describe the process of assigning a chiral centre as R or S
- Rank the four substituents on the chiral centre in order of priority. Higher atomic numbers get higher priority. You are ranking the first atom attached to the centre.
- Orientate the molecule so that the lowest priority substituent points away from you
- Join the other three substituents with an arrow from highest to lowest priority.
- If the arrow points clockwise, the centre is R. If it is anticlockwise the centre is S
Describe E and Z isomers
Any alkene which has two different substituents attached to each carbon of the double bond can exist as two different geometrical isomers, known as E and Z isomers
Describe the process of determining whether an isomer is E or Z
- Assign the priorities of the two substituents at each carbon of the double bond separately.
- If the higher priority substituents are on the same side of the double bond (ie both at the top or both at the bottom), then it is the Z-isomer. If they are on opposite sides, it is the E-isomer.
State the name for stereoisomers which are not enantiomers
Diastereoisomers, or diastereomers
Give the general rule for the number of stereoisomers and enantiomers in a compound with n chiral centres.
In general, a compound with n chiral centres will have 2n stereoisomers, consisting of 2(n-1) pairs of enantiomers.
Describe the use of a double-headed curly arrow
A double-headed curly arrow is used to depict the movement of two electrons. The tail of the arrow shows where the electrons come from, and the head of the arrow shows where the electrons go.
Describe 4 general rules for drawing mechanisms
- A chain of curly arrows can start at a negatively charged atom, an uncharged atom with a lone pair, a double or a single bond
- At intermediate positions, we make a bond to an atom and break a bond to the same atom, not add and take electrons.
- At the start of the chain, electrons are removed, and at the end of the chain, electrons are added
- The charge is always conserved.
The phenomenon in which a molecule's true structure cannot be depicted by a single structure, but instead is depicted as an average of two or more 'canonical forms', is called resonance
Resonance is an effect which influences the distribution of electrons in molecules.
Describe 3 rules for drawing resonance structures
- Canonical forms only differ by the position of the electrons, not the nuclei
- The energy of the molecule is lower than that of any of the canonical forms
- Canonical forms don't necessarily contribute equally to the true structure of the molecule. Low energy structures contribute the most and visa versa.
State and describe Hückel's rules
If a system fulfils Hückel's rules, it will be aromatic
- Fully conjugated (no sp3 hybridised atoms)
- Has (4n+2)π electrons where n is an integer
Describe a conjugated system
A conjugated system is one of connected p orbitals with delocalised electrons
Describe what resonance effects in a molecule
Resonance only involves π electrons and lone pairs and is a stabilising effect.
State the suffix and prefix that monosubstituted benzenes are named with
State the nomenclature of a benzene ring with substituents at the 1st and 2nd positions
State the nomenclature of a benzene ring with substituents at the 1st and 3rd positions
State the nomenclature of a benzene ring with substituents at the 1st and 4th positions
State the catalyst needed for the halogenation of a benzene ring
FeBr3 or AlCl3
State the catalyst needed for the nitration of a benzene ring
H2SO4 and HNO3
State the catalyst needed for the sulfonation of a benzene ring
SO3 and H2SO4
Describe a Friedel-Crafts Alkylation
R-Cl and AlCl3 are reacted with benzene to attach the R group onto the benzene ring
Describe a Friedel-Crafts Acylation
O=R-Cl and AlCl3 are reacted with benzene to attach the R=O group (ie the acyl group) to the benzene ring
Give the four categories that substituents can be sorted into
- Mesomerically electron donating
- Inductively electron donating
- Electron withdrawing
Describe one method of drawing enantiomers
Swap dashed and solid wedges around
Define a meso - compound
A meso - compound is a non optically active stereoisomer. Meso compounds are symmetric around their stereogenic centre