Week 7 Flashcards
Carbon’s electrons
Shares 4 electrons to form 4 covalent bonds
* 2 x 2s electrons + 2 x 2p electrons are available
* But these orbitals (2s + 2p) mix (hybridise) first
sp3 hybridisation
If the 2s orbital mixes with all 3 2p orbitals…
* (1 x s) + (3 x p) = 4 x sp3
* I.e. 4 x identical orbitals -> forms 4 x single (covalent) bonds
sp2 hybridisation
If the 2s orbital only mixes with 2 2p orbitals..,
* (1 x s) + (2 x p) = 3 x sp2 (+1 x p-orbital remains unhybridized)
* I.e 3 x identical hybridised orbitals + 1 different one -> forms 3 x single bonds + 1 x double bond
Properties of alkanes
- Chemically inert (i.e. don’t do a whole lot!)
- Do react with oxygen (combustion) -> used as fuels
- short-chain alkanes burn efficiently CO2 + H2O
- bigger alkanes don’t burn as well ( sooty flame)
Constitutional isomers
Isomers that differ in the nature or sequence of bonding
* Within a homologous sequence of alkanes, the number of constitutional isomers increases rapidly
Stereoisomers
Isomers which differ in arrangement of groups in space (but same nature and sequence of bonding)
Two subsets:
* configurational
* conformational
Configurational isomers
Interconverted only by breaking and remaking bonds.
* This process normally requires a lot of energy so does not happen at room temperature
Conformational isomers (conformers)
Differ by rotation about a single (C-C) bond
* Easy at room temperature so conformational isomers are not normally separable
A double bond is includes a
sigma bond and a pi bond
To rotate around the C–C axis would require…
The p-bond to break
* takes 128 kJ mol-1 and does not occur without significant energy input
Double bond isomers
If both ends of the C=C bond each have two different groups attached then:
* two diastereoisomers result
Z or E diasteroisomers
- If high priority groups on same side of C=C ⟹ Z double bond
- If high priority groups on opposite sides of C=C ⟹ E double bond
The higher the atomic number of the atom attached…
the higher the priority
Rhodopsin
Pigment found within the retina of our eyes
* Within it is retinal - respond to light coming in
In ‘straight chain’ alkanes, rotation about C-C bonds is ____ at room temperature
Rapid
* Differences in energy arise from steric interactions
Configurational isomers
can also arise when…
an sp3 atom has 4 different bonding partners
Chirality
An object is chiral if it is NOT superimposable upon its mirror image
Enantiomers
- Are mirror images of each other
- Are not superimposable (i.e. not the same).
- Commonly contain one or more stereocenters
stereocenter
stereogenic centre = chiral centre
stereocenter = carbon atom with four different groups attached
racemic mixture (racemate)
50:50 mixture of both enantiomers
Each individual chiral centre has two possible arrangements
Enantiomers labelled as R or S
When Does Chirality Matter?
Chiral molecules occur frequently with increasing molecular size
* Almost all the physical properties of a pair of enantiomers are identical (melting point, boiling point, solubility, NMR, IR etc.)
* Behave in an identical way when interacting with achiral objects or reagents
* Will interact & react differently with other chiral molecules
Almost all biological catalysis by enzymes is selective for a particular enantiomer.
True,
Almost all proteins and enzymes are chiral
Measuring optical activity
When plane polarised light is passed through a solution of one pure enantiomer of a compound, the plane of polarisation is rotated:
* Enantiomer is said to be optically active
* Amount of rotation (⍺) is characteristic of the enantiomer
One enantiomer causes clockwise rotation, the other enantiomer rotates the light an equal amount in the opposite direction
- rotation in a clockwise direction is labelled (+)
- rotation anti-clockwise is labelled (-)
- racemic mixtures give an overall rotation of zero
Diasteroisomers
Configurational isomers that are NOT enantiomers
* All achiral config. isomers are diasteromer
each chiral centre has…
2 possible arrangements
