Alkenes, Benzenes, SN1/SN2 and E1/E2 Flashcards
Lectures 14-20
General formula
CnH2n
Geometry of alkenes
- contain C=C double bond
* Geometry of carbons is trigonal planar (bond angle of 120°)
Describe the E/Z nomenclature of alkenes
- assign priorities
- if the high priority groups are on the:
SAME side - (Z)
OPPOSITE sides - (E)
• sides are divided horizontally by the double bond
Describe the stability of different alkenes
- The more substituted the alkene, the more stable it is.
* disubstituted is more stable than monosubstituted
Requirements for aromaticity
- cyclic
- conjugated (Alternating double and single bonds)
- planar
- Huckel’s rule: 4n + 2 ∏ electrons where n=integer
Disubstituted benzenes nomenclature
1,2 - Ortho
1,3 - meta
1,4 - para
pKa values and the strength of acid
Lower the pKa value, the stronger the acid
What are the two types of bond breaking?
- homolytically: Each atom of bond gets one electron to form 2 radicals (odd electron species)
- heterolytically: Atom of bond with higher electronegativity gets 2 electrons to form 2 even electron species
What are the two types of bond making?
- Homogenic: 2 radicals donate one electron to form a bond
* heterogenic: two electrons donated from one species to another to form a bond
Nucleophile
• Electron rich molecules or ions that possess a non-bonding lone-pair of electrons (a.k.a Lewis Bases)
Electrophiles
- Electron deficient molecules or ions
* usually lack a stable octet and can accept a pair of electron (a.k.a Lewis Acids)
How is electronegativity related to nucleophile/electrophiles?
- delta + : less electronegative, electrophilic
* delta - : more electronegative therefore nucleophilic
Electron density maps
- Illustrates the electron density of the regions in the molecule
- red - high density = nucleophilic
- Purple - low density = Electrophilic
Transition state
Highest energy state along the reaction coordinate
Polarity
Unsymmetrical distribution of electrons in a bond due to difference in electronegativities
Nucleophilic substitution reaction
Replacement of a leaving group by a nucleophile
• involves a leaving group (usually the molecule with higher electronegativity)
What are the 2 types of nucleophilic substitution reactions?
SN1 - stepwise (2 step reaction)
SN2 - concerted (simultaneous, one step reaction)
What are the factors which determine if reaction is SN1 or SN2?
- nature of alkyl group
- nature of leaving group
- nature of nucleophile
Rate of SN1 reaction
rate = k x [R-X]
Rate only depends on the concentration of the substrate
Rate: 3° > 2° > 1°
• Tertiary carbocation is most stable therefore fastest rate
Describe an SN1 reaction
- Ionisation: leaving group leaves (Slow step)
- nucleophilic attack
- involves a carbocation intermediate
- End up with a racemic mixture
- weak nucleophiles
Rate of SN2 reaction
Rate = k x [R-X][Nu]
Rate depends on concentration of both the substrate and nucleophile
Rate: 1° > 2° > 3°
• Tertiary has most steric hindrance
SN2 reaction
- breaking and making of bond is simultaneous
- Nucleophile approaches from a direction 180 deg away from leaving group
- Walden inversion - only end up with one enantiomer if we start with one enantiomer
What is an optically pure sample?
A sample that contains only one enantiomer
SN1 or SN2 reaction for tertiary carbons?
SN1 preferred
• intermediate is very stable
• tertiary is too sterically hindered for SN2
SN1 or SN2 reaction for primary carbons?
SN2 preferred
• sterically accessible
• not SN1 because resulting carbocations are unstable
What is special about secondary substrates?
Can undergo either SN1 or SN2 reactions and it is often difficult to predict which is more likely
Are weak bases better nucleophiles or leaving groups?
- Weaker bases are usually better leaving groups
- Stronger bases are better nucleophiles
Strong bases attack to displace the weaker base
What is an elimination reaction?
The loss of an atom or group resulting in the formation of a multiple bond.
What is the main difference between elimination and substitution reactions?
- usually, basicity parallels nucleophilicity
- for elimination, we require strong bases that are relatively weak nucleophiles
e.g. strong base can be a weak nucleophile due if it has high steric hindrance
E1 reaction steps
- ionisation - leaving group leaves (Slow step) and forms a carbocation
- deprotonation - by losing a proton, carbocation becomes a more stable species
E2 reaction
• breaking and making of bonds is simultaneous or concerted
Describe the products of an elimination reaction
Elimination reactions can result in more than one product
• Major products: The ones that are ‘more substituted’ and hence more stable
• Minor product
disubstituted - double bond carbons attached to 2 carbons
How does temperature influence the type of reaction that occurs?
• High temperatures - 100℃
Elimination reactions are more likely
• Low temperatures - 0℃
Substitution reactions more likely
What reaction is mostly likely to occur if H2O is the nucleophile?
- H2O is a weak nucleophile
- therefore can only attack an unstable carbocation
- carbocations are formed in 2 step reactions (SN1 or E1)
- E1 for hot, and SN1 for cold
What reaction is most likely to occur between a tertiary haloalkane and OH-?
- OH is a strong base
- so it is either, SN2 or E2
- tertiary haloalkane - high steric hindrance for nucleophile to approach it, so SN2 cannot occur
What reaction is most likely to occur with t-Bu as a nucleophile?
- t-Bu is a strong base hence a strong nucleophile (SN2 or E2)
- due to steric hindrance it cannot coordinate an SN2 reaction
- therefore an E2 reaction occurs
Is Br2 a nucleophile or electrophile?
- When Br2 is approached by a nucleophile, it becomes polarised
- one side is delta + and another is delta -
- therefore it can react with a nucleophile, hence is considered as an electrophile
What is Zaitsev’s rule?
In the elimination fo H-X from an alkyl halide, the more highly substituted, and less sterically crowded alkene product predominated (major).
• In practise, this means there will be as few H atoms as possible bonded to C=C bond.
How to determine whether substitutions or elimination will occur?
Bulky nucleophile - elimination
• A bulky attacking nucleophile suffers more steric hindrance attacking an electrophile in substitution than elimination
Secondary, tertiary alkyl halide more likely to undergo elimination than primary
• In a substitution reaction, electrophilic carbon atom being attacked is more sterically hindered in secondary or tertiary - so elimination occurs
What are good leaving groups?
- ones that can be displaced easily
* strong bases are bad leaving groups (i.e. OH-)
When will secondary haloalkanes undergo SN1 or SN2 reactions?
SN1 favoured in
• protic solutions - ones that contain a H proton that can be easily displaced/donated (ie. OH, NH3, H2O)
• with weak nucleophiles
SN2 favoured in
• aprotic solutions - cannot donate hydrogen
• with strong nucleophiles
Why are polar protic solvents favoured by SN 1 reactions?
- polar protic solvents have hydrogen atoms connected to electronegative atoms, and can be easily donated
- in sn1 reactions a carbocation intermediate is formed
- leaving group becomes a anion
- the H atom in the polar protic molecules can stabilise both the carbocation and anion (leaving group)
Why are polar aprotic solvents favoured by SN2 reactions?
• SN2 reactions are one-step reactions involving a strong nucleophile
• the polar aprotic solvents is able to stabilise the cation formed by the nucleophile
i.e. in nucleophile NaOH, Na+ will be stabilised to increase the ability of OH to attack
