Alkenes✅ Flashcards
How is a pie bond formed
By the sideways overlap of two p-orbitals, one from each C atom of the double bond
Where is the electron density of the pie bond, what does the pie bond also do in regards to rotation
Concentrated above and below the line joining the nuclei of the bonding atoms
Pie bond locks the 2 C atoms in position and prevents them from rotating around the double bond
What is the bond angle in an alkene
120
State what is meant by unsaturated (1 mark)
Compound with one or more double or triple bonds
Explain the difference between a sigma and pie bond (3 marks)
A sigma bond is result of the head on overlap of orbitals whereas a lie bond is formed from the sideways overlap of p orbitals
Why is the bond angle around the C atom in an alkene different to an alkane (3 marks)
In alkene there are 3 bonding regions about the C atom, these repel equally resulting in trigonal planar. In alkane 4 bonding pairs of electrons, these repel equally in a tetrahedral shape, trig planar is 120, tetrahedral is 109.5
What is a stereoisomer
Same structural formula but different arrangement of the atoms in space
Definition: molecules with the same structural formula but a different arrangement of the atoms in space
When will a E/Z isomer occur
A C=C double bond
Different groups attached to each C atom of the double bond
What is the difference between a E and Z isomer
E, if the groups of higher priority are diagonally placed across the double bond
Z, group of higher priority are in same side of the double bond
What does cis and trans isomer equate to in terms of E/Z isomer
Cis=Z
Trans=E
What determines the priority in an E/Z isomer
Atomic number is bigger has a higher priority if it’s the same it’s what comes after it
Why are alkenes much more reactive than alkanes
Presence of pie bond, the pie electrons are more exposed than sigma bond electrons, a pie bond readily breaks and alkenes undergo addition reactions relatively easily
What is the bond enthalpy of a sigma and pie bond
Sigma is 347KJmol-1
Pie is 265kJmol-1
Give examples of alkenes that undergo addition reactions with
Hydrogen in presence of nickel catalyst
Halogens
Hydrogen halides
Steam in presence of acid catalyst
What conditions does there need to be for hydrogenation of alkenes, give an example with propene
423K, passed over a nickel catalyst
Propene becomes propane
Give an example with propene and bromine for the halogenation of alkenes
Propene + bromine= 1,2-dibromopropane
How do you test for unsaturation
Bromine water added dropwise to sample the oranje color disappears as double bone broken
Conditions for addition reactions of alkenes with hydrogen halides give an example of propene with hydrogen chloride
Alkenes react with gaseous hydrogen halides at room temp, if alkene is a gas reaction takes place when 2 gases mixed, if alkene liquid hydrogen halide bubbles through
Propene + HCl= 1-chloropropane or 2-chloropropane
What are the conditions and give an example of propene with hydration reactions of alkenes
Alcohols formed when alkenes react with steam in presence of a phosphoric acid catalyst
Propene + steam under presence of phosphoric acid catalyst forms propan-1-ol or propan-2-ol
Why is the mechanism called electrophilic addition
Double bond in alkene represents region of high electron density cause of presence of pie bond electrons
High electron density of pie electrons attract electrophiles (atom or group of atoms attracted to an electron rich center and accepts an electron pair)
Describe the electrophilic addition mechanism with bromine and but-2-ene
1: br more electronegative than hydrogen, so H bromide is polar and contains dipole of H+ and br -
2: electron pair in our hind attracted to partially positive H cause double bond to break
3: bond forms between H atom of H-Br molecule and a C atom that was part of double bond
4: the H-Br bond breaks by heterolytic fission, with electron pair going to bromine atom
What is the most stable carbocation
Tertiary, secondary, primary
What determines the major product
Carbocation stability
What is a repeat unit
The specific arrangement of atoms in polymer molecule that repeats over and over again
Written in square brackets with n bottom right corner
