definitions for prelim

Question 1

Molecular orbitals?

Answer 1

Molecular orbitals form when atomic orbitals combine

Question 2

Pi bonds

Answer 2

Molecular orbitals that form by side-on overlap of parallel atomic
orbitals that lie perpendicular to the axis of the covalent bond.

Question 3

Sigma bonds

Answer 3

Molecular orbitals that form by end-on overlap of atomic orbitals
along the axis of the covalent bond.

Question 4

Hybridisation

Answer 4

Hybridisation is the process of mixing atomic orbitals within an atom
to generate a set of new atomic orbitals called hybrid orbitals

Question 5

sp^3 orbitals

Answer 5

the 2s orbital and the three 2p orbitals of carbon hybridise
to form four degenerate sp3 hybrid orbitals

Question 6

sp^2 orbitals

Answer 6

The 2s orbital and two of the 2p orbitals hybridise to
form three degenerate sp2 hybrid orbitals

Question 7

sp orbitals

Answer 7

The 2s orbital and one 2p orbital of carbon hybridise to form two
degenerate hybrid orbitals

Question 8

HOMO

Answer 8

highest occupied molecular orbital (HOMO)

Question 9

LUMO

Answer 9

The lowest antibonding
molecular orbital is called the lowest unoccupied molecular orbital
(LUMO).

Question 10

Chromophore

Answer 10

A chromophore is a group of atoms within a molecule that is responsible for absorption of light in the visible region of the spectrum

Question 11

Conjugated system

Answer 11

conjugated system —
a system of adjacent unhybridised p orbitals that overlap side-on to
form a molecular orbital across a number of carbon atoms. Molecules with
alternating single and double bonds, and aromatic molecules have
conjugated systems

Question 12

Homolytic bond fission

Answer 12

• results in the formation of two neutral radicals
• occurs when each atom retains one electron from the sigma covalent
  bond and the bond breaks evenly
• normally occurs when non-polar covalent bonds are broken

Question 13

Heterolytic bond fission

Answer 13

• results in the formation of two oppositely charged ions
• occurs when one atom retains both electrons from the sigma
  covalent bond and the bond breaks unevenly
• normally occurs when polar covalent bonds are broken

Question 14

Single headed arrow

Answer 14

a single-headed arrow indicates the movement of a single
electron

Question 15

Double headed arrow

Answer 15

double-headed arrow indicates the movement of an electron
pair

Question 16

Nucleophiles

Answer 16

• negatively charged ions or neutral molecules that are electron
  rich, such as
  Cl^- , Br^- , OH^- , CN^- NH3 and H2O
• attracted towards atoms bearing a partial positive or full positive charge
• capable of donating an electron pair to form a new covalent bond

Question 17

Electrophiles

Answer 17

• positively charged ions or neutral molecules that are electron
  deficient, such as H^+ NO2^+ and SO3
• attracted towards atoms bearing a partial negative or full negative
  charge
• capable of accepting an electron pair to form a new covalent bond

Question 18

Skeletal formula

Answer 18

skeletal structural formula, neither the carbon atoms, nor any
hydrogens attached to the carbon atoms, are shown

Question 19

Monohaloalkanes

Answer 19

• contain only one halogen atom
• can be classified as primary, secondary or tertiary according to
  the number of alkyl groups attached to the carbon atom
  containing the halogen atom

Question 20

SN1

Answer 20

SN1 is a nucleophilic substitution reaction with one species in the rate
determining step and occurs in a minimum of two steps via a trigonal
planar carbocation intermediate.

Question 21

SN2

Answer 21

SN2 is a nucleophilic substitution reaction with two species in the rate
determining step and occurs in a single step via a single five-centred,
trigonal bipyramidal transition state

Question 22

Alcohols

Answer 22

Alcohols are substituted alkanes in which one or more of the
hydrogen atoms is replaced with a hydroxyl functional group, –OH
group

Question 23

Ethers

Answer 23

Ethers can be regarded as substituted alkanes in which a hydrogen
atom is replaced with an alkoxy functional group, –OR, and have the
general structure R’ – O – R’’, where R’ and R’’ are alkyl groups.

Question 24

Markovnikov’s Rule

Answer 24

Markovnikov’s rule states that when a hydrogen halide or water is
added to an unsymmetrical alkene, the hydrogen atom becomes
attached to the carbon with the most hydrogen atoms attached to it
already. Markovnikov’s rule can be used to predict major and minor
products formed during the reaction of a hydrogen halide or water
with alkenes.

Question 25

Amines

Answer 25

Amines are organic derivatives of ammonia in which one or more hydrogen atoms of ammonia has been replaced by an alkyl group

Question 26

Benzene

Answer 26

Benzene (C6H6) is the simplest member of the class of aromatic hydrocarbons. The benzene ring has a distinctive structural formula. The stability of the benzene ring is due to the delocalisation of electrons in the conjugated system. The presence of delocalised electrons explains why the benzene ring does not take part in addition reactions. Bonding in benzene can be described in terms of sp2 hybridisation, sigma and pi bonds, and electron delocalisation

Question 27

Isomers

Answer 27

Molecules that have the same molecular formula but different structural formulae

Question 28

Structural isomers

Answer 28

Structural isomers occur when the atoms are bonded together in a different order in each isomer

Question 29

Stereoisomers

Answer 29

Stereoisomers occur when the order of the bonding in the atoms is the same but the spatial arrangement of the atoms is different in each isomer. There are two types of stereoisomer, geometric and optical.

Question 30

Geometric isomers

Answer 30

- can occur when there is restricted rotation around a carbon to carbon double bond or a carbon-carbon single bond in a cyclic compound - must have two different groups attached to each of the carbon atoms that make up the bond with restricted rotation - can be labelled cis or trans according to whether the substituent groups are on the same side (cis) or on different sides (trans) of the bond with restricted rotation -have differences in physical properties, such as melting point and boiling point -can have differences in chemical properties

Question 31

Optical isomers

Answer 31

- occur in compounds in which four different groups are arranged tetrahedrally around a central carbon atom (chiral carbon or chiral centre) - are asymmetric - are non-superimposable mirror images of each other - can be described as enantiomers -have identical physical properties, except for their effect on plane polarised light - have identical chemical properties, except when in a chiral environment such as that found in biological systems (only one optical isomer is usually present) - rotate plane-polarised light by the same amount but in opposite directions and so are optically active - when mixed in equal amounts are optically inactive because the rotational effect of the plane-polarised light cancels out — this is called a racemic mixture

Question 32

racemic mixture

Answer 32

when mixed in equal amounts are optically inactive because the rotational effect of the plane-polarised light cancels out

Question 33

Experimental microanalysis

Answer 33

Elemental microanalysis is used to determine the masses of C, H, O, S and N in a sample of an organic compound in order to determine its empirical formula.

Question 34

Mass spectrometry

Answer 34

Mass spectrometry can be used to determine the accurate gram formula mass (GFM) and structural features of an organic compound.

Question 35

The mass-to-charge ratio

Answer 35

The mass-to-charge ratio of the parent ion can be used to determine the GFM of the molecular ion, and so a molecular formula can be determined using the empirical formula.

Question 36

Infrared spectroscopy

Answer 36

Infrared spectroscopy is used to identify certain functional groups in an organic compound

Question 37

Proton nuclear magnetic resonance spectroscopy

Answer 37

Proton nuclear magnetic resonance spectroscopy (proton NMR or 1H NMR) can give information about the different chemical environments of hydrogen atoms (protons or 1H) in an organic molecule, and about how many hydrogen atoms there are in each of these environments.

Question 38

High-resolution 1H NMR

Answer 38

High-resolution 1H NMR uses higher radio frequencies In a high-resolution 1H NMR an interaction with 1H atoms on neighbouring carbon atoms can result in the splitting of peaks into multiplets

Question 39

low-resolution 1H NMR

Answer 39

low-resolution 1H NMR and provides more detailed spectra

Question 40

Agonist

Answer 40

An agonist mimics the natural compound and binds to the receptor molecules to produce a response similar to the natural active compound.