Block 4 Flashcards
(51 cards)
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
