Block 4 Flashcards
Describe the functional group alkenes: what types of bonds, what is hybridisation of the carbons, bond angle and orientation?
two carbon-carbon bonds, one is sigma and one is pi, both carbons are sp2 hybridised so double bond is planar with bond angles of ~120 degrees
The alkene double bond is considered to be: … with 2 electrons in the … bond and 2 electrons in the … bond
electron rich, sigma and pi
State the two reactions that prepare alkenes
elimination reactions - acid catalysed (H2SO4) dehydration of alcohols and base promoted (KOH/NaOH in ethanol) removal of HX from alkyl halides.
What is Saytzeff rule
The major product is the most substituted alkene; that is the alkene with the least number of hydrogens directly attached to the carbons of the C=C
Why is the major product of elimination favoured?
Less hydrogens attached to the Cs in the double bond means that is it is a lower energy/ stabler product
Why do alkenes generally undergo addition reactions
the double bond acts as a nucleophile as it is e- rich
Describe hydrogenation of alkene (sometimes called reduction)
alkene + H2 - Pt catalyst -> alkane
What is the stereochemistry of hydrogenation of alkene
Syn - Hs are added to the same side of the alkane forming cis- alkanes
Halogenation has the reagents of Br2, Cl2. What is the mechanism of addition
- Halogen acts as an electrophile with the electrons in the alkene double bond polarising the Br-Br bond. Double bond attacks the Br forming a carbocation +Br
- The remaining halide ion acts as a nucleophile and adds to the carbocation intermediate
What is the stereochemistry of halogenation of an alkene and why is it that way
Anti-stereochemistry = only gives the trans product. This is because the carbocation intermediate is actually a bromonium ion intermediate that has Br bonded to the 2 carbons so that the second Br can only attack from the top
If there are other nucleophiles present at part 2 of the halogenation mechanism these can
compete with the other halide ion and give different products.
Markovnikovs rule: Addition of an unsymmetrical reagent (H-Z) to an unsymmetrical alkene gives what as the major product
the compound where the electropositive part of the reagent (usually H+) has bonded to the carbon onf the C=C that is directly bonded to the greater number of hydrogen atoms.
describe the addition of HZ to alkene mechanism
- double bond attacks H+ forming carbocation.
2. addition of the nucleophile to the C+ and then charges are balanced by loss of proton if required
Why is the major product of addition favoured?
the major product is formed from the more stable carbocation which is in order from 3>2>1. More stable= more readily formed so more of the product formed.
Describe the function group of alkynes : what types of bonds, what is hybridisation of the carbons, bond angle and orientation?
2 sp carbons with one sigma bond and 2 pi bonds with bond angles of 180 °
Describe how alkynes are prepared : reaction, reagents, mechanism
dehalogenation of a dihaloalkane using NaOH/ KOH in ethanol. 2 moles of HX are removed to form an alkyne.
Benzene (an aromatic compound) does not undergo reactions typical of alkenes (addition) because of
The conjugated double bonds in the structure of benzene giving it extra stability = resonance energy. Electrons are shared around the ring.
Cyclic hydrocarbons can be aromatic if they contain
4N+ 2 e where N is a while number
How do you say monosubstituted benzenes
substituent name + benzene = bromobenzene
what is phenol
benzene + alcohol
What is ortho
1,2
What is meta
1,3
What is para
1,4
How do you make an alkane from an alkyne
add pt or palladium + H2
How do you make a Z alkene (Hs on the same side) from alkyne
add lindlar catalyst and H2
How do you make an E alkene (Hs added trans) from alkyne
add Li/liq ammonia and then water
How do you make a ketone from alkyne
using aqueous H2SO4 or HgSO4. IT adds one mole of water
What is the reaction mechanism of addition of water (hydration) to alkyne
Markovnikov addition of OH and H which is unstable and forms a tautometric equilibrium between that (the enol) and the ketocompound which is the final product.
How to form an alkynide anion and why does it happen
A strong base can remove the hydrogen on the terminal carbon of an alkyne as the terminal sp carbon is weakly acidic.
Forming an alkynide anion is useful for
carbon chain extension
What is an electrophile
E+, something that loves electrons. e- attack it
Describe the mechanism of electrophilic aromatic substitution (SeAr)
- double bond attacks the electrophile.
- wayland intermediates—> resonance stabilised cations form
- Proton loss
What is the fast and slow step of SeAr
fast step is proton loss. slow step is electrophile attack.
Describe whats happening to the charge in the resonance stabilised cation. What is another name for this
Moving an electron to the positive charge that forms when two things are bonded to a carbon in the ring. The positive charge is only ortho or para to the Electrophile, never meta. Wayland intermediates equates to Resonance hybrid. Has a dotted half circle moon with plus charge in the middle.
What is the stereochemistry of electrophilic (HZ or X2) addition reaction of an alkyne
Markovnikoffs rule is followed for HX addition, the reaction can be stopped after one addition and anti stereochemistry of addition is observed
Why does a G substituent on the benzene ring direct ortho/para. What are the substituents that are op directing
It has lone pairs of electrons that can be donated.
OH, NH2, OCH3, X or R
Why does a G substituent on the benzene ring direct meta. What are the substituents that are meta directing
It is electron withdrawing and has a positive end of a polar bond or a formal negative charge.
CN, COOH, COH, NO2
What are the activating G substituents and why are they activating
OH, NH3,-OCH3 and R. They have electrons to donate by resonance. R donates them by induction
What are the deactivating substituents and why are they deactivating
X, CN, COH, COOH and NO2 because they are electron withdrawing - the electropositive part of the polar bond is attached to the benzene.
What is the explanation of substituent directing power for ortho para directors.
When an orthopara director is attached it has 4 resonance contributors for ortho/para but only 3 for meta. so more resonance contributors = more stable.
What is the explanation of substituent directing power for meta directors.
For meta directors they have their positive end bonded to the benzene making only 2 resonance contributors possible (positive charge cannot be on the carbon bonded to the substituent for ortho/para but 3 for meta substituion.
Tell 2 reactions that make haloalkanes
HX addition to an alkene (adding anti or alcohol with SOCl2 which swaps the OH for a Cl.
What is the order of increasing ease of substitution for halogens
The bigger halogen it is the easier it is to leave -> starting from Cl, Br, I = most easy
What is the rate order for Sn1 and E1 reactions of a haloalkane and what other thing do they have in common
rate is first order because the mechanism is 2 steps by a carbocation intermediate with the first step only having the RX as the reactant.
What is the stereochemistry of a Sn1 mechanism
Sn1 mechanism have racemic mixtures of S and R enantiomers when the haloalkane is chiral.
What is the stereochemistry of a Sn2 mechanism
For Sn2 using chiral but non racemic haloalkane results in the products becoming the opposite R/S due to inversion of configuration at the site of substitution
The reaction order usually favours the more stable carbocation so tertiary haloalkanes for elimination and substitution of haloalkane. Which mechanism is different
Sn2 because transition state is less crowded for 1 and 2 so Ea is lower.
What is the rate law for bimolecular mechanisms E2 and Sn2
rate = [RX][Nu-]
Describe the mechanism of E2 elimination of a haloalkane
The base attacks the H, removing the proton, the C=C is formed and the C-X bond is broken all in one step.
What are the rules for filling out the table telling you about the competing reactions
- Sn2 fills the top two rows with single Sn2 being the T
- E2 is only in basic column
- E1 is only in the bottom row
- Sn1 pairs with E1 and Sn2 of 2’
Grignard reagents are made using
aryl /akyl halide + Mg in dry diethylether to make a carbon + Mg + halide which makes the carbon a carbanion
