Lecture 10 Flashcards
(18 cards)
3 classes of reaction arrows
- Lone pair → bond
- Bond → lone pair
3.bond → bond
Class 1 lone pair → bond
The first example : a lone pair on a hydroxide ion going to hydronium ion (H+). The arrow shows the
formation of a bond between O and H, with electrons from the lone pair on the oxygen.
- Arrow A shows: lone pair → bond
“Tail”: Oxygen is going from owning an electron pair to sharing it. So it “loses” an electron, resulting in it becoming more positive by 1.
From unshared pair of electrons on oxygen atom to a point between the hydrogen and oxygen atoms in the
structure, unshared pair of electrons on oxygen moves between the oxygen and hydrogen to form covalent
bond.
“Head”: Hydrogen is going from lacking to sharing. So it “gains” an electron, resulting in it becoming
more negative by 1
Class 2 bond → lone pair
The second example shows the reverse reaction: dissociation of water to give H+ and a hydroxide ion.
Here, the arrow shows the breaking of the O-H bond, to end up as a lone pair on oxygen.
- Arrow B shows: bond → lone pair
“Tail”: Hydrogen is going from sharing an electron pair to lacking it. So it “loses” an electron, resulting in it becoming more positive by 1. The electrons in the covalent bond between the oxygen and hydrogen are moved onto the oxygen atom.
“Head”: Oxygen is going from lacking an electron to owning it. So it “gains” an electron, resulting in it becoming more negative by 1
Class 3 bond →bond
Finally, an example of a bond breaking to form another bond. Here, the arrow shows the breaking of a π bond between C1 and C2, and the formation of a new C–H bond between C1 and H
- Arrow C shows: bond → bond
“Tail”: C2 is going from sharing an electron pair to lacking it. So it “loses” an electron, resulting in it
becoming more positive by 1
“Head”: Hydrogen is going from lacking an electron to sharing it. So it “gains” an electron, resulting in it becoming more negative by 1.
Types of organic reactions
- Addition
- Elimination
- Substitution
- Rearrangement
Electrophilic addition
Polar reactants can be classified as either electrophiles or nucleophiles
• The electrons are more exposed to an attacking reagent than are the electrons
• The bond is weaker than bond and the pi electrons are involved in addition of alkenes
• The double bond can act as a supplier of pi electrons to an electron seeking reagent
E+ + :Nu- E:Nu
Addition reactions
Alkenes are quite reactive because of the double C=C bond
In addition reactions:
• a group A of the reagent A – B becomes attached to one of the carbon atom of the double bond (C C),
• group B becomes attached to the other carbon atom
• the product then has only single bonds between carbon atoms
Addition of halogens (halogenation)
Alkenes readily add chlorine and bromine (even at room temp, but usually require heat or UV light)
Addition of Br2
Addition of hydrogenation
Hydrogenation of cyclohexene
Addition of water (hydration)
Hydration is the addition of water to an Alkene to form an alcohol.
Addition of acid to alkynes
Basic pattern of an addition reaction:
• Break C-C multiple bond (
• Form two new single bonds to carbon (sigma to carbon)
• Occurs only at the carbons that are part of multiple bonds, nothing else is affect
Reaction mechanism
A Reaction mechanism is a step by step description of the bond-breaking and bond- making process that occur when reagent react to form products
Reaction Mechanism of Electrophilic Addition
The proton H+ is attacking electrophile.
As the proton approaches the bond, the
two pi electrons are used to form bond.
And then this C-H bond uses 2 pi electrons, the
other carbon acquires a positive charge +
Carbocation - is a positively charged carbon atom,
attached to three other atom
Reaction mechanism 2
The carbocation (very reactive) combine with some species and (which) can supply it with two electrons
Summary of the Reaction Mechanism
Attack of Electrophile – Formation of Carbocation – Attack of Nucleophile – Formation of the Product
Example: Alkene reacts with HBr
The hydrogen bromide molecule is polarised (bromine has a higher electronegativity than hydrogen).
1. Attack by the electrophile opening the double bond and producing an intermediate carbonium ion and leaving a negative bromide ion
2. The bromide ion then joins to the positive carbon atom of the carbocation (carbonium
Markovnikov’s Rule
• With an unsymmetrical alkene, HX can add to the double bond to give two constitutional isomers, but only one is actually formed:
• This is a specific example of a general trend called Markovnikov’s rule.
• Markovnikov’s rule states that in the addition of HX to an unsymmetrical alkene, the H atom adds to the less substituted carbon atom—that is, the carbon that has the greater number of H atoms to begin with
Markovnikov’s Rule
• The basis of Markovnikov’s rule is the formation of a carbocation in the rate-determining step of the mechanism.
• In the addition of HX to an unsymmetrical alkene, the H atom is added to the less substituted carbon to form the more stable, more substituted carbocatio
Classification of carbocations, their stability
Carbocations are classified as primary, secondary, tertiary when one two or 3 other groups are attached, respectively
Methyl
Primary
Secondary
Tertiary
Carbonium ion stability - induction
Primary carbonium ion: Induction from on alkyl group only
Secondary carbonium ion: Induction from two
alkyl group results in better stabilisation than primary
Tertiary carbonium ion: Induction from three alkyl group gives even better stabilisation
