module 3 Flashcards
Isomers?
Different compounds with the same molecular formula
Constitutional isomers?
Isomers with a different order of attachment
Stereoisomers \?
Same order of attachment of atoms, but different orientation of their atoms in space.
Enantiomers?
Stereoisomers that are nonsuperimposable mirror images of each other.
Diastereoisomers?
Stereoisomers that are not mirror images.
How to identify E and Z isomers?
Assign priorities to the molecules on the left and right sides of a carbon double bond. If the highest priority atoms are on opposite sides of the molecule (top and bottom) the isomer is E (trans) and alternatively same side would be Z (cis).
What Letter is assigned if an enantiomers configuration is clockwise?
R Configuration.
What Letter is assigned if an enantiomers configuration is anti-clockwise?
S Configuration
What is a homolytic bond cleavage?
ONE electron is transferred to each of the TWO atoms. The species generated (X. and Y. are radicals.
What is a heterolytic bond cleavage?
TWO electrons are transferred from the bond to ONE atom. The species generated are charged intermediates.
When simply bonded functional groups are attached, why is the c-x bond polar?
The X group is more electronegative that C.
The C atoms is electrophilic (seeking electrons).
What are Nucleophiles?
Nucleophiles are species that can donate a pair of electrons to the positive carb and form a bond.
Describe general nucleophilic substitution reactions?
If X is a good leaving group (C-X bond is weak) then when a nucleophile approaches the c-x bond will break and the nucleophile will form a new bond with the C atom.
Describe general elimination reactions?
Occurs in slightly more complex molecules (more than one carbon atom. Nucleophiles can also act as bases. When it approaches it will remove a proton (Hydrogen atom) from the molecule, those electrons are transferred to the C atom resulting in the formation of a Pi bond (double bond).
What is an Sn2 reaction?
Nucleophilic Substitution
X leaves and Nu approaches
One step reaction
Reactants - transition state - products
Explain the transition state of an Sn2 reaction?
Effective collision has taken place
Corresponds to maximise free energy
Nu and X are both loosely bound to C
If activation energy is high the reaction will be slow
What is a Sn1 reaction?
Nucleophilic substitution
X leaves, then Nu attaches
Two step reaction
Slow step - Transition state (Reaction intermediate) - fast step - deprotonation.
Explain the slow step of an Sn1 reaction?
Removal of leaving group (independent of nucleophile)
Bond stretched to breaking point (transition state)
Explain fast step in Sn1 reaction?
Addition of nucleophile
Explain the Transition state of Sn1 reaction?
Carbocation produced from slow step acts as the reaction intermediate. It is very reactive and will react with any nucleophile and major product will be from from reaction with most abundant nucleophile.
Stereochemistry of Sn2 reaction?
Nu approaches opposite to leaving group
Sn2 reaction is stereospecific
Stereochemistry of Sn1 reaction?
Nu can approach and attach to a planar reaction intermediate (carbocation) from either pole.
Occurs with inversion and retention of stereochemistry.
What types of molecules go via Sn2?
Methyl, Primary, Secondary (can do either)
What types of molecules go via Sn!?
Secondary (can do either) , Tertiary
What determines the rate of Sn1?
Can be fast if changes in transition state are stabilized
.
What determines the rate of Sn2?
Can be fast if approach of nucleophile is not sterically hindered.
Eg collision with C or too many groups attached to C.
In both pathways what step does the loss of leaving group occur in?
Rate determining step.
How do the two mechanisms differ in terms of leaving group requirements?
For Sn2 a strong Nu can make up for a poor leaving group but for Sn1 a good leaving group is essential.
Can some compounds be converted into better leaving groups?
Yes, HO and RO under strong acidic conditions (protonation) because the +ve charge pulls the electrons in the c-o bond towards the oxygen, weakening the bond.
Example of a good and poor leaving group?
OH = good H20 = poor
What stabilizes the transition state in Sn1 reactions?
H20 being a polar solvent
polar effect of C
H bonding to L
What determines Elimination reaction mechanisms?
base strength.
Strong base = E2
Weak base = E1
What is an E1 mechanism?
Elimination reaction
Two step reaction
Often co-occurs with Sn1 reactions
Explain first step of E1?
Rate determining step
Same as Sn1
generates are carbocation intermediate
Explain second step of E1?
A base (H20 most abundant) removes H atom to form c=c bond.
What is an E2 mechanism?
Elimination reaction
One step reaction
Explain. step one of E2 mechanism ?
Concerted bond breaking and bond formation.
Accessible H+ is removed
Halide and Base involved in single rate determining step
Must have strong base
Possible for primary, secondary, and tertiary
What is an Addition reaction?
Alkene carbocation doubled bond reacts with nucleophile and electrophile.
Dissociation of reactant into Nu and E followed by Two step reaction.
Explain first step of Addition reactions?
Slow step - addition of electrophile to the electron rich double bond perpendicular to the atoms plane.
(Rate determining step)
Explain Second step of Addition reactions?
Fast step - attack by nucleophile of the carbocation intermediate. (Carbocation intermediate will react with the most abundant nucleophile.)
