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Organic Chemistry Week 1 Summary - Chapter 5


Enantiomers and the tetrahedral carbon
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Enantiomers are optical isomers (identical properties but rotate light in the opposite direction)

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Achiral objects are superimposable with their mirror images

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Chiral objects are not superimposable

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Mirror images are called enantiomers

Optical activity – assigning direction of rotation
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Levorotatory are molecules that rotate light to the left and ar denoted using a (-)

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Dextrorotatory molecules are molecules that rotate light to the right and are denoted by (+)

Cahn-Ingold-Prelog sequencing rules
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Order elements by atomic umber to assume highest priority

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If the carbon is not chiral look at substituents until difference is found

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Multiple-bonded atoms are equivalent multiple single bonded atoms

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If clockwise rotation of substituents it is labelled R enantiomer

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If anticlockwise is labelled S enantiomer

Diastereomers
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Molecules with more than one chirality centre

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Are not mirror images

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Have same configuration at some chirality centres and different at others

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Two diatereomers that are the same at all chirality centres but different at one

Epimers
Mesocompounds
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Have a plane of symmetry

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Achiral but have two chiral centres

Racemic mixture
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Mixture with equal portions of levorotatory and dextrorotatory isomers and show no optical
rotation

Prochiral molecule – is an achiral molecule that can be transformed into a chiral molecule in a single step

Priority in chirality is also in the order to atomic number
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If it is counter clockwise it is the Si face
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If the chiral centre has 3 different groups and the last
can be substituted to make a chiral carbon it is considered prochiral
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Meaning that
the pro-S atom in the molecule when substituted will lead to an R chirality centre
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Chapter 6

How organic reaction occur:
There are two ways a two-electron covalent bond can break:
Symmetrical or Homolytic: One electron stays with each product fragment

Unsymmetrical or Heterolytic: Both electrons stay with one fragment leaving the other with a vacant orbital


Polar Reactions:
O, N, F, Cl more electronegative than C therefore carbon is more delta positive when attached to these
When carbon is attached to metal element it is delta negative
Polar bonds can also result from acid base reactions
Nucleophile – negatively polarised and electron rich It forms a bond by donating a pair of electrons and may be
neutral or negatively charged
Electrophile – is electron loving and will accept electrons to become more negative
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Alkenes
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Contain a double bond

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Sigma parts of Sp2-sp2 overlap

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Pi parts of p-p overlap

Double bonds
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Greater electron density than single bonds

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Electrons in pi bonds more accessible to approaching reactants

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Is nucleophilic and reacts with an electrophile



Polar reaction mechanisms
Rule 1 – electrons move from the nucleophile to the electrophile
Rule 2 – the nucleophile can be negative or neutral







Rule 3 – the electrophile can be positive or neutral (the negative charge must be stabilised on the electronegative
atom








Rule 4 – The octet rule must be followed (CNOF must be left with 8 electrons and H left with 2)





Describing a Reaction:
The value of the Keq in a reaction tells which side of the reaction is energetically favoured
If the Keq is greater than 1 the product is favoured so reaction proceeds left to right
If the Keq is = 1 both products are present at equilibrium
If the Keq is less than 1 the reactant is favoured and the reaction proceeds from right to left
For a reaction to have a favourable equilibrium constant the reaction must proceed from left to right
Bond dissociation energy (D) is the amount of energy required to break a given bond to produce two fragments



∆G˚ - the energy difference between the reactants and the products
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The transition state represents the highest-energy structure involved in the reaction, it is unstable and cannot be
isolated
Activation energy – the energy difference between the original energy state (the reactants energy state) and the
transition state, determines how rapidly a reaction will occur at a given temperature
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It is more stable than
the transition state and can be isolated for short periods of time




For biological reactions to occur at physiological conditions they must have:
- low activation energy
- release energy in small amounts
(catalyst will cause the reaction to occur faster by proceeding with smaller steps)
Biological processes cannot be reproduced in a lab because they are too complex and too fast and inside cells it
is an aqueous liquid and high specificity for substrate, within a lab it is an organic liquid and low specificity for
substrate
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Name parent hydrocarbon
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Find longest carbon chain containing double bond

Step 2
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Write the full name with suffix –ene instead of –ane
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If there is more than one double bond use suffixes like diene triene





Cycloalkenes - number the cycloalkane so the double bond is between 1 and 2 and the substituent has the lowest
number possible

Alkynes
Named main chain with triple bond received lowest number
Named with the suffix yne

When there is a choice in numbering enynes the double bond always gets the lowest number if bother are
between the last carbons on either side

In a carbon-carbon double bond the carbon are sp2 hybridised and have a planar conformation with a pi bond
above and below and a sigma bond connecting
Cis-Trans Isomerism in Alkanes

Alkene E,Z designation
1
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Determine which substituent is higher priority

Alkenes become more stable with increasing substitution

Electrophilic Addition Reactions of Alkenes
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Carbon-carbon double bonds act as a nucleophile (lewis base) in polar reactions by donating a
pair of electrons to an electrophile (lewis acid)

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Successful with H2O, HCL, HBr and HI

Worked example:

Alkynes also undergo electrophilic addition reactions but their reactivity is substantially less

Regiospecific reactions: Where only one of two possible orientations occur

Markovnikov’s rule dictates where each substitute in the reaction is placed
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Hydride shift: The shift of a hydrogen atom and its electron pair between neighbouring carbons, the secondary
carbocation intermediate formed by protonation rearranges to form a more stable tertiary carbocation

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