Block 4: Functional Groups I Flashcards
Why are alkenes good nucleophiles?
The double bond holds four electrons between 2 carbons, giving them a high electron density. Therefore they are attracted to nuclei and can react as a nucleophile
What are the 3 ways of preparing an alkene?
Elimination.
- Catalytic dehydrogenation
- Acid-catalysed dehydration
- Base promoted dehydrohalogenation
What reagent is needed for calatlytic dehydrogenation of an alkane to form an alkene?
Platinum catalyst and at lease 60 degree C heat
What reagent is needed for acid-catalysed dehydration of an alcohol to form an alkene?
Conc. H2SO4
What reagent is needed for base promoted dehydrohalogenation of a haloalkane to form an alkene?
A strong base (KOH etc) in alcohol
What is Zaitsev’s rule?
The major product is the most substituted alkene- the alkene with the least number of hydrogens directly attached to the carbons of C=C (poor get poorer)
What are the reactions alkenes can undergo?
Hydrogenation and reactions using H+ as an electrophile
What is the reagent to hydrogenate an alkene to an alkane?
H2 and a Pt Catalyst
In what stereochemistry does the H2 add to the alkene?
Syn stereochemistry- same side of the molecule
What are the steps of hydrogenation of an alkene on the Pt catalyst?
- 2 H atoms are adsorbed onto the catalyst.
- The alkene arrives and complexes with the catalyst temporarily and non-covalently.
- The H and C next to each other bond, leaving an empty bond for the other H to attach to.
- The alkane is produced and the catalyst is regenerated.
How does halogenation occur?
When X2 is added, it acts as an electrophile, where one X bonds to the old double bond and breaks its covalent bond. This forms a carbocation and an X anion. Then, the other X atom bonds to the now positive C+ charge. This only occurs anti as the first X takes up all the space on its side of the molecule, preventing the other X to bind syn. (NB with bromine, this is called a bromonium ion intermediate).
What should be remembered about halogenation of an alkene?
If other nucleophiles are present they can compete to give a different product.
Discharge of Br2 color is used as a lab test to detect alkenes.
In what reagents does H+ act as an electrophile when adding to alkenes?
HCl to form monosubstituted haloalkane.
H+/OHR to form Ether
H+/H2O to form Alcohol.
What is markovnikov’s rule?
Addition of an unsymmetrical reagent to an unsymmetrical alkene gives as the major product the compound in which the electropositive part of the reagent (H+) has bonded to the carbon of the C=C directly bonded to the greater number of H atoms (rich get richer)
What is the reaction mechanism for the addition of HZ to an alkene?
- H+ is added across the C=C bond, forming a carbocation on the carbon it does not bond to.
- The nucleophile (Z) attaches to the carbocation. If it is H2O/H+, the whole H2O molecule adds before the extra H+ is lost to form an alcohol
Why does markovnikov’s rule work in terms of carbocations?
When there are two possible products from an addition reaction, the C with more Cs attached to it has less Hs. Therefore, a 2nd or 3ry carbocation will be formed, which is more stable than the 1my one it could have formed. Therefore it is more likely to remain in solution, and be the dominant product.
How do you prepare an alkyne?
Didehydrohalogenations of dihaloalkanes. The reagent used is alcoholic OH-. Firstly a single HX is removed, resulting in a vinyl halide. Finally the second HX is removed, allowing an alkane to form.
What reactions do alkynes undergo?
Hydrogenation, Electrophilic addition of HX and X2, Hydration and formation of alkynide anions (on terminal alkynes).
What products can alkynes form when they undergo hydrogenation?
They can form alkanes, E-alkenes or Z-alkenes.
How do akynes form alkanes?
With addition of 2H2 and a Pt or Pd catalyst
How do alkynes form E-alkenes?
With addition of Li and Liquid NH3 and then H2O
How do alkynes form Z-alkenes?
With a lindlar catalyst (Pd/Pb(OAc)2)/H+)
What needs to be remembered about electrophilic addition of HX or X2 to alkynes?
- Markovnikov’s rule is followed
- The reaction can be stopped after 1 mol of reagent (to make an alkene) or let all react (for an alkane)
- Anti stereochemistry of addition is observed.
What needs to be remembered about hydration of alkynes?
- Only one mol of water reacts. Therefore the product always has a double bond.
- For all alkynes except ethyne, the product is a ketone (in ethyne the product is an aldehyde).