13.3/13.4 Reactions of Alkenes and Electrophilic addition Flashcards
(20 cards)
Why are alkenes more reactive than alkanes?
-C=C’s pi bond’s electron density being concentrated above and below the sigma bond
(The electrons are more exposed)
-Sigma bond enthalpies are stronger than pi bonds
(TL;DR the pi bond break more readily)
What substances will alkenes undergo addition reactions with?
-Hydrogen (in the presence of nickel catalysts)
-Halogens
-Hydrogen Halides
-Steam (in the presence of an acid catalyst)
How do you test for unsaturation?
When orange bromine water is added dropwise to an alkene sample, the liquid will lose its colour.
What happens to the double bonds in alkenes when they undergo addition reactions?
The pi bonds creating the double bond are broken
What is an electrophile?
An electron-deficient species that accepts electrons
What is a hydration reaction?
When a water molecule is added to another molecule
What is produced when an alkene undergoes hydration?
An alcohol
What is created when an alkene reacts with a halogen/hydrogen halide?
A halogenoalkane
What is created when an alkene reacts with hydrogen gas (in the presence of a nickel catalyst) ?
An alkane
Are all double bonds hydrogenated when there is an excess of hydrogen in dienes/trienes etc. ?
Yes
What is created when an alkene reacts with steam (in the presence of a sulphuric acid catalyst) ?
An alcohol
How does the mechanism for electrophilic substitution work in alkenes?
1.) In a permanent dipole, the partially positive/induced dipole in non-polar molecules will be attracted to the double bond in an alkene
2.) This causes the double bond in the alkene to break and a new bond is formed with the partially positive atom
3.) The other reagent’s bond breaks by heterolytic fission. (Both electrons go to the partially negative atom)
4.) An anion and a carbocation are formed
5.)The ion and the carbocation create the final product
What is a carbocation?
An positively charged carbon atom
How do you pronounce carbocation?
Car-bow-cat-ion
Where do you draw the curly arrows in each step of the mechanism?
Initial Step.) The arrow starts on the double bond and points to the partially positive atom. Another arrow starts in the bond of the other re-agent and points to the partially negative ion.
Intermediate Step.) An arrow starting from the lone pair (or negative charge) and points to the carbocations positive charge
What is Markownikoff’s rule?
When hydrogen halides react with an asymmetrical alkene, the hydrogen atom will attach itself to the carbon with the greatest number of hydrogen atoms and least amount of carbon atoms
What are minor products?
A minor product is an isomer of a smaller yield created in a reaction
What are major products?
An isomer of a larger yield created in a reaction.
What causes the yield difference in isomers in addition reactions?
Halides/partially negative ions will mostly attach to the more stable carbocation
What is the trend of stability in carbocations?
(Most to least stable)
1.) Tertiary
2.) Secondary
3.) Primary
