Chapter 13 - Alkenes Flashcards
What is the general formula of an alkene?
CnH2n
What is a pi bond?
The sideways overlap of 2 p orbitals, one from each carbon atom of the double bond - each C atom contributes one electron to the electron pair in the pi bond.
How does a pi bond affect the geometry of an alkene?
The pi bond locks the two C atoms in position, preventing them from rotating around the double bond
What is the shape and bond angle around a double bond, and why?
Trigonal planar, 120 degrees
Due to 3 regions of electron density around each carbon atom, which repel as far as possible
What is a stereoisomer?
Structures with the same molecular formula, but with a different arrangement of the atoms in space
Why is there stereoisomerism around a double bond?
Because rotation around the double bond is restricted and the groups attached to each carbon atom are therefore fixed relative to each other
What conditions must a molecule satisfy to demonstrate E/Z isomerism?
- a C=C double bond
- different groups attached to each carbon atom of the double bond
What is the difference between cis-trans isomerism and E/Z isomerism?
Cis-trans isomers must have one of the attached groups on each carbon atoms as the same
How do you decide if a molecule is a Z or E isomer?
If groups of higher priority are on same side of double bond, the compound is Z isomer
If groups of higher priority are diagonally placed across the double bond, the compound is E isomer
How do you assign priority to the substituent groups for E/Z isomerism?
The higher the atomic number, the higher the priority.
If the two atoms attached to a carbon atom in the double bond are the same, you have to find the first point off difference - whichever has higher atomic number is higher priority
Why are alkenes more reactive than alkanes?
Due to their pi bond - which more readily breaks as the electrons are more exposed than the electrons in a sigma bond. It is easier to break pi bond than sigma bond
What are the conditions required when alkenes undergo hydrogenation, and what products are formed?
Conditions: nickel catalyst, 423K
Addition reaction forms alkane as hydrogen added across double bond
What are the conditions required when alkenes undergo halogenation, and what products are formed?
Conditions: room temp
Forms haloalkane as double bond opens up for diatomic halogen molecule
How can bromine water be used to test for unsaturation?
Bromine adds across the C=C double bond in alkenes and bromine water (usually orange) decolourises.
What are the conditions required when alkenes undergo addition with hydogen halides, and what products are formed?
Conditions: room temperature
Forms haloalkanes
What are the conditions required when alkenes undergo hydration, and what products are formed?
Conditions: phosphoric acid catalyst
Steam adds across the double bond to form alcohol
What is an electrophile?
An atom/group of atoms that is attracted to an electron rich centre and accepts an electron pair
Why are electrophiles attracted to alkenes?
The C=C double bond represents a region of high electron density due to the pi electrons
Describe the mechanism for reaction between but-2-ene and hydrogen bromide
- Bromine is more electronegative than hydrogen, so HBr is polar and has a dipole
- Electron pair in pi bond attracted to partially positive H atom, causing double bond to break
- Bond forms between H atom of HBr molecule and carbon atom that was part of double bond
- HBr bond breaks by heterolytic fission, with electron pair going to bromine atom
- Br- ion and carbocation formed
- Br- ion reacts with carbocation to form addition product
What is Markownikoff’s rule?
The most stable carbocation (based on the attachment of R groups) will form the major products of electrophillic addition
What is the order of stability for carbocations?
Primary = least stable, one R group Tertiary = most stable, 3 R groups
What is a polymer?
An extremely large molecule formed from many thousands of repeat units of smaller molecules known as monomers
What is a repeat unit?
The specific arrangement of atoms in the polymer unit that repeats over and over again (always written within square brackets)
What is (poly)ethene’s common uses?
Supermarket bags, shampoo bottles, children’s toys
