Organic

Question 1

Briefly describe covalent bonding

Answer 1

Covalent bonds form most of the bonds in organic compounds. When forming covalent bonds, atoms tend to attain a noble gas configuration

Question 2

State the three things to consider when looking at how covalent bonds form

Answer 2

• The total number of electrons in an atom
• The number of electrons in the outer shell
• The valency (ie the number of bonds)

Question 3

Define alkynes

Answer 3

A homologous series containing at least one carbon-carbon triple bond

Question 4

Define allenes

Answer 4

Compounds in which one carbon atom is double bonded to two other carbon atoms

Question 5

Define the node/nodal plane

Answer 5

An area of low electron density

Question 6

Describe how to count electrons for any atom in a molecule

Answer 6

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

Question 7

Describe s orbitals

Answer 7

S orbitals are spherical

Question 8

Describe p orbitals

Answer 8

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.

Question 9

Describe hybrid orbitals

Answer 9

When bonds are formed, various combinations of s and p orbitals give hybrid orbitals, which define the shapes of molecules

Question 10

Describe sp3 hybridisation

Answer 10

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

Question 11

Describe the bonding in methane

Answer 11

Each sp3 hybrid orbital forms a sigma bond (a region of high electron density) with a hydrogen atom.

Question 12

Describe sp2 hybridisation

Answer 12

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.

Question 13

State the multiplier given by ‘pico’

Answer 13

1x10^-12

Question 14

Define a double bond and state its bond length in carbon

Answer 14

A double bond is formed of one sigma bond and one pi bond. A carbon carbon double bond is approximately 134pm

Question 15

Describe sp hybridisation

Answer 15

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.

Question 16

Define a triple bond and state its bond length in carbon

Answer 16

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.

Question 17

Describe how to identify sp hybridisation by looking at the number of pi and sigma bonds

Answer 17

If a carbon atom has 2 pi bonds and 2 sigma bonds, it is sp hybridised

Question 18

Describe how to identify sp2 hybridisation by looking at the number of pi and sigma bonds

Answer 18

If a carbon atom has 1 pi bond and 3 sigma bonds, it is sp2 hybridised

Question 19

Describe how to identify sp3 hybridisation by looking at the number of pi and sigma bonds

Answer 19

If a carbon atom has 0 pi bonds and 4 sigma bonds, it is sp3 hybridised.

Question 20

State which out of pi and sigma bonds are weaker

Answer 20

Pi bonds are weaker than sigma bonds.

Question 21

Define the electronegativity of an element

Answer 21

Electronegativity is a measure of an element’s tendency to attract the electron density in a covalent bond

Question 22

Describe what happens in a bond if there is a difference in electronegativity between the atoms

Answer 22

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

Question 23

Describe how positive charges are affected in molecules with more than two atoms

Answer 23

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

Question 24

Describe the inductive effect, using H-CH2-Cl as an example

Answer 24

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

Question 25

List substituents in order of strongest electron withdrawing ability to strongest electron donating ability

Answer 25

* Nitro groups * Fluorine * Chlorine * Bromine * Iodine * Alcohols & ethers * Hydrogen * Metals

Question 26

Explain why alcohols and ethers are lower in the list of electron withdrawing ability than may have been expected

Answer 26

The effect from the oxygen atom is ‘diluted’ as it is working in two directions, not one.

Question 27

State what is meant by drawing a solid wedge

Answer 27

The substituent at the end of a solid wedge is pointing towards you, out of the plane

Question 28

State what is meant by drawing a dashed wedge

Answer 28

The substituent at the end of a dashed wedge is pointing away from out, into the plane

Question 29

Describe enantiomers

Answer 29

Enantiomers are a pair of compounds which are non-superimposable mirror images of one another. They are a form of optical isomers

Question 30

Describe a chiral molecule

Answer 30

A chiral molecule is one that is not superimposable on its mirror image

Question 31

Describe a chiral centre

Answer 31

A chiral centre is most commonly a carbon atom with four different groups attached to it. It can also be called a stereogenic centre

Question 32

Describe the process of assigning a chiral centre as R or S

Answer 32

* 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

Question 33

Describe E and Z isomers

Answer 33

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

Question 34

Describe the process of determining whether an isomer is E or Z

Answer 34

1. Assign the priorities of the two substituents at each carbon of the double bond separately. 2. 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.

Question 35

State the name for stereoisomers which are not enantiomers

Answer 35

Diastereoisomers, or diastereomers

Question 36

Give the general rule for the number of stereoisomers and enantiomers in a compound with n chiral centres.

Answer 36

In general, a compound with n chiral centres will have 2ⁿ stereoisomers, consisting of 2^(n-1) pairs of enantiomers.

Question 37

Describe the use of a double-headed curly arrow

Answer 37

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.

Question 38

Describe 4 general rules for drawing mechanisms

Answer 38

1. 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 2. At intermediate positions, we make a bond to an atom and break a bond to the same atom, not add and take electrons. 3. At the start of the chain, electrons are removed, and at the end of the chain, electrons are added 4. The charge is always conserved.

Question 39

Define resonance

Answer 39

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.

Question 40

Describe 3 rules for drawing resonance structures

Answer 40

1. Canonical forms only differ by the position of the electrons, not the nuclei 2. The energy of the molecule is lower than that of any of the canonical forms 3. Canonical forms don't necessarily contribute equally to the true structure of the molecule. Low energy structures contribute the most and visa versa.

Question 41

State and describe Hückel's rules

Answer 41

If a system fulfils Hückel's rules, it will be aromatic 1. Cyclic 2. Planar 3. Fully conjugated (no sp³ hybridised atoms) 4. Has (4n+2)π electrons where n is an integer

Question 42

Describe a conjugated system

Answer 42

A conjugated system is one of connected p orbitals with delocalised electrons

Question 43

Describe what resonance effects in a molecule

Answer 43

Resonance only involves π electrons and lone pairs and is a stabilising effect.

Question 44

State the suffix and prefix that monosubstituted benzenes are named with

Answer 44

...benzene phenyl...

Question 45

State the nomenclature of a benzene ring with substituents at the 1st and 2nd positions

Answer 45

Question 46

State the nomenclature of a benzene ring with substituents at the 1st and 3rd positions

Answer 46

Question 47

State the nomenclature of a benzene ring with substituents at the 1st and 4th positions

Answer 47

Question 48

State the catalyst needed for the halogenation of a benzene ring

Answer 48

FeBr₃ or AlCl₃

Question 49

State the catalyst needed for the nitration of a benzene ring

Answer 49

H₂SO₄ and HNO₃

Question 50

State the catalyst needed for the sulfonation of a benzene ring

Answer 50

SO₃ and H₂SO₄

Question 51

Describe a Friedel-Crafts Alkylation

Answer 51

R-Cl and AlCl₃ are reacted with benzene to attach the R group onto the benzene ring

Question 52

Describe a Friedel-Crafts Acylation

Answer 52

O=R-Cl and AlCl3 are reacted with benzene to attach the R=O group (ie the acyl group) to the benzene ring

Question 53

Give the four categories that substituents can be sorted into

Answer 53

1. Mesomerically electron donating 2. Inductively electron donating 3. Electron withdrawing 4. Halogens

Question 54

Describe one method of drawing enantiomers

Answer 54

Swap dashed and solid wedges around

Question 55

Define a meso - compound

Answer 55

A meso - compound is a non optically active stereoisomer. Meso compounds are symmetric around their stereogenic centre