Chapter 13 Flashcards
What are alkenes?
unsaturated hydrocarbons that contain one or more carbon double bond
CnH2n
-2 double bonds= diene suffix
-120 degrees
-Trigonal planar
- all atoms are on the same plane
What do bonds require?
Orbital overlap
-e.g. Sigma bonds are a head-on overlap of orbitals (single bonds)
S orb - S orb overlap // P orb- P orb overlap // S orb - P orb overlap
What is an orbital?
An area with a high likelihood of encountering an electron, holding a maximum of 2 electrons in each orbital
What are Pi bonds? and double bonds?
sideways overlap of 2 p orbitals
a double bond is one sigma and one pi bond
- p orbitals sit at 90 degrees to the plane of the molecule to form the pi bond
If there is an alcohol group, what bond angle is there?
104.5 because the oxygen has two bonded pairs and two lone pairs, with lone pairs repelling more than bonded pairs
=> 104.5 degrees so it is non linear around the oxygen atom
What are stereoisomers
same structural but different arrangement of atoms in space
E/Z isomerism
-caused by the C=C double bond (groups attached to each carbon atom are fixed relative to each other and cannot rotate)
- ONLY works if it satisfies both conditions
=> a double bond and different groups attached to each carbon atom of the double bond
E isomers and Z isomers
E isomers form when the different groups are diagonal to each other (zigzag skeletal)
Z isomers form when the groups are on the same side as each other (U shaped skeletal= flat double bond)
Cis/trans isomerism
a special type of E/Z where each carbon in the double bond MUST be attached to hydrogen
In this case, Cis = Z and Trans = E
Cahn-Ingold-Prelog nomenclature
atoms attached to each carbon in the double bond are given priority based upon their atomic number
- If groups of higher priority are on the same side, compound is Z ISOMER
- If groups of higher priority are diagonally placed, compound is E isomer
The higher the atomic number, the greater the priority
- If two atoms attached to a carbon atom are the SAME, then the first point of difference must be looked at
Reactivity of alkenes
More reactive than alkanes because of the presence of the pi bond
=> P- electron density is concentrated ABOVE and Below the plane of sigma bond
=> Since it is outside double bond, the pi electrons are more exposed than electrons in sigma bond. Therefore a pi bond readily breaks and alkenes undergo addition reactions relatively easily
=> bond enthalpy of a pi bond is less than a c-c sigma bond
Addition of hydrogen to alkenes (Hydrogenation)
nickel catalyst and 423K (150 celsius)
-breaks double bond and adds hydrogen across double bond
Addition of halogens to alkenes (halogenation)
reaction works best at room temperature
Addition of alkenes with hydrogen halides
room temperature
- alkene as gas is mixed with gaseous hydrogen halide/ alkene as liquid means gaseous hydrogen halide is bubbled through
- can also work if alkene is combined with concentrated HCl or HBr
Addition of H20 (Hydration)
steam (H2O gas) plus phosphoric acid (H3PO4)/ Sulphuric acidd (H2SO4)
-makes an alcohol
What is an electrophile
an electron pair acceptor (an atom/ group of atoms that are attracted to electron rich centres and can accept electron pairs)
-usually positive ions/molecules that contain a partial positive charge
What is a nucleophile
an electron pair doner (atom/group of atoms that is attracted to an electron deficient centre, to which it can donate a pair of electrons
- usually molecules and ions with a free pair of electrons or a pi bond
Mechanism for electrophilic addition of Hydrogen Bromide
- Bromine is more electron negative than Hydrogen so HBr is polar (Bromine is delta negative and hydrogen is delta positive)
- Electron pair in Pi bond (double bond) is attracted to partially positive hydrogen, causing the double bond to break (arrow starts from double bond and goes to delta positive hydrogen
- HBr breaks as a bond between the hydrogen and carbon atom so a heterolytic fission occurs with the electron pair going only to bromine atom
- Carbocation forms (a positively charged carbon atom) and Br- bromide ion forms
- Bromide ion is attracted to the carbocation and forms the addition product
electrophilic addition of Bromine
Only difference is that Br2 is non polar yet when it approaches an alkene, the pi bond electrons interact with the electrons in the Br-Br bond
–> repels electron of Br-Br, causing one side to be partially positive and the opposite side to be partially negative (induced dipole)
Carbocation stability
Each alkyl group will donate and push electrons towards the positive charge of the carbocation
=> therefore, if there are more alkyl groups, the positive charge will spread throughout the molecule more and make it more stable
They can be of 3 types:
1. Primary (only 1 alkyl group)
2. Secondary (2 alkyl groups)
3. Tertiary (3 alkyl groups)
The more carbon atoms there are around the carbocation, the more stable it is
Markownikoff’s rule
When a hydrogen halide reacts with an unsymmetrical alkene, the hydrogen of the hydrogen halide attaches itself to the carbon atoms of the alkene with the greater number of hydrogen atoms and smaller number of carbon atoms
