C Unit 2.5 Hydrocarbons Revamp Flashcards
What are the advantages of fossil fuels?
(Pick and roll. bruh)
- Cheap, easily available
- Releases lots of energy
- Available 24hrs a day
- ” in all states of matter
Disadvantages of fossil fuels?
(Piiiick and roooolllll)
- Finite/non-renewable
- Releases CO2 (global warming)
- Produces SO2 (acid rain)
The word eqn of combustion of alkanes?
Alkane + oxygen -> carbon dioxide + water
Hence, symbol eqn of combustion of alkanes?
(Varies based on alkane… octane ig?)
C8H18 + 25O2 -> 8CO2 + 9H2O
What are alkanes?
(4 points)
- Saturated compounds
- Composed of C and H atoms
- w/ σ bonds
- Homologous series w/ formula CnH2n+2
3 properties of alkanes?
(+ 3 small explanations)
- Strong stable bonds, direct head-on overlap
- Free rotation, σ bonds allow it
- Non-polar molecules, C-H/C-C similar electronegativities
Consequently, alkanes “net effect”?
(Overall outcome due to its properties)
Fairly restricted set of reactions
What are 3 rxns u can do with alkanes?
(+ 3 small explanations)
- Burn them, destroy whole molecule
- React w/ halogens, break C-H bonds
- Crack them, break C-C bonds
Electronic configuration of an alkanes’ (carbon) four outermost electrons?
+ significance?
2s2, 2p2
, all used in bonding
How come all 4 electrons in carbon (alkanes) used in bonding?
(5-way)
- Sp3 hybridisation
- From 2s2, 2p2
- To 2s1, 2p3
- 1 of the 2s e-</sup< gets promoted
- Now has 4 unpaired electrons
Explain CH4 tetrahedral shape?
(2-way)
- 4 Sp3 hybrid orbitals repel each other equally
- Guess they 109.5° apart
What are free radicals?
(2-way)
- Atoms/groups of atoms
- Having single unpaired e-
What type of reaction involves free radicals?
Free radical substitution reaction
I mean… then what type of mechanism is halogenation?
(Probably optional but aye depends)
… Free radical mechanism
How can one form free radicals?
(2-way)
Name for that?
- Bond splitting evenly
- Each atoms getting 1/2 electrons
- Homolytic fission
Although, what’s another term for homolytic fission?
Photochlorination
The 3 stages of homolytic fission
Stage 1 - Initiation
Stage 2 - Propagation
Stage 3 - Termination
Explain initiation stage… typically
(2-way)
- UV radiation causes cl-cl bond (or any halogen perhaps)
- to split homolytically
Write an initiation stage example?
(Cl2)
Cl]-[Cl -> Cl˙ + Cl˙
-> = UV
˙ dot = radical
Explain propagation stage?
(4-way)
- Radicals are very reactive
- n take part in series of propagation rxns
- Radical is a reactant, process will form radicals as a product
- Chain reaction pretty much
Write/”draw” a propagation stage example?
(CH4 w/ Cl)
CH4 + Cl˙ -> ˙CH3 + HCl
then
˙CH3 + Cl2 -> CH3Cl + ˙Cl
, there’s even a mechanism drawing here but meh.
Explain termination stage
Propagation continues until 2 radicals meet = TERMINATION
Write either 3 termination stage examples?
Cl˙ + Cl˙ -> Cl2
Cl˙ + ˙CH3 -> CH2Cl
˙CH3 + ˙CH3 -> CH3CH3
How are alkenes more reactive than alkanes?
(4-way)
- Alkene = C=C bond
- More electron density (=bond = π and σ bond… 2 e- pairs)
- Can use that additional e- pair to form new bonds
- Alkanes break if their σ-bond is used despite already being used
What are alkenes?
(4-way)
- Unsaturated compounds
- Composed of C-H & C=C bonds
- w/ σ + π bonds
- General formula of CnHn times 2
3 properties of alkenes?
(+ 3 small explanations)
- Stronger than alkanes, the C=C bond
- Restricted rotation, due to π bond
- More reactive, additional e- pair usage
What hybridisation does alkene have?
+ significances
(5-way)
- Sp2 hybridisation
- 1 2s orbital + 2 2p orbitals = 3 Sp3 hybrid orbitals
- Remaining unhybridised 2p orbital unchanged
- Results in trigonal planar shape + σ bonding (hybridised)
- π bonding (unhybridised)
Hence, what creates the alkenes double bond?
+ additional significances in between
(5-way)
- σ bond from head on overlap of 2 Sp3 hybrid orbitals
- strong bond holding the Cs together
- π bond from sideways overlap of 2 unhybridised p orbitals
- Sideways ∴ weaker
- Electron density above + below plane of molecule
Draw ethene’s bonds
+ explanation (2-way)
- Stage is yours
- Due to electrons overlapping
- giving rise to π bonds (or can opt for other explanation)
Due to alkenes restricted rotation, what is used to distinguish different orientations containing perhaps functional groups?
E and Z notations
E meaning in E/Z notation?
Opposite side
(Entgegen)
Z meaning in E/Z notation?
“Z”ame side
(Zussamen)
For E/Z notation, how to tell which is first in the name?
(2-way)
- Priority rule
- One with highest atomic mass
Define electrophile?
(4-way)
- Atom/group of atoms attracted to
- electron-rich centre/atom
- Accepting pair of e-
- to form a new covalent bond
Examples of an electrophile?
(Yup.)
- HBr
- HCl
- Cl2
- Br2
- H2
Define nucleophile?
(3-way)
- Chemical (don’t overthink lol)
- that can donate pair of e- w/
- subsequent formation of a covalent bond
Examples of a nucleophile?
(Mhm.)
- -OH
- Br-
- Cl-
- -CN
- H2O
- -NH3
So which are susceptible to be attacked by an electrophile?
… alkenes, and only alkenes.
Hence, the mechanism for electrophiles attacking?
Electrophilic addition
What is electrophilic addition often associated with?
(What is this, trivia?)
Heterolytic bond fission
What 4 things do u need for a “model” mechanism
- Curly arrows
- Dipoles
- Lone pairs
- Charges
Draw “model” mechanism of rxn between ethene and HBr
Stage is youuuurrrsss
What 2 other “addition” reactions are to of our concern?
- Bromination
- Hydrogenation
Describe bromination?
(1 + 2-way)
- Test for alkenes
- Bromine water decolourises
- if double bond present
Draw bromination mechanism
(Ethene + Br2)
Just ye lol
Describe hydrogenation?
(2 things)
- ‘Saturates’ alkenes by adding hydrogen
- Different variations of product can be produced (maybe)
Draw hydrogenation mechanism
(Propene + H2)
Go go goooo
Define carbocation?
(4-way)
- Carbon atom having a positive charge
- due to electron deficiency
- Species highly reactive
- Defined by ⊕︀
Draw reaction mechanism between HCl and propene
Show which is produced in greatest quantities
(2 products formed)
Bruh
From the reaction mechanism between HCl and propene, why is one produced in greatest quantities?
(2-way + 2-way)
- 2° carbocations more stable than 1°
- due to more alkyl groups attached
- Which those donate e- density
- towards carbocation to stabilise it
Show stability of the carbocations?
(From less to most)
3° > 2° > 1°
2 tests you can use for alkenes?
- Bromination
- Using potassium manganate (VII)
What conditions needed to test alkenes using KMnO4?
Must be cold.
(Alkenes react if cold w/)
Difference in observations for acidified/slightly alkaline KMnO4?
(Each has the changes)
- Acidified = purple solution -> colourless
- Slightly alkaline = purple solution -> dark green -> dark brown precipitate
Although, what’s the jig of using KMnO4?
(1 + 3-way)
(As long as it’s understood)
- May contains carbon-carbon double bond
- Equally could be any one of a
- large n° of other compounds
- oxidised by manganate(VII) ions under alkaline conditions
Define addition polymerisation?
(3 beefy points wat)
- Addition reaction that continues until actively stopped
- Monomers adding together to form long chain polymers
- Only used w/ alkenes & substituted alkenes
What does a polymer not contain? and how is the name formed?
(Last is 2-way)
- Doesn’t actually have double bonds
- By adding poly at prefix,
- to involve -ene (bruh)
Draw ethene turning into polyethene
Case close it
From drawing ethene turning into polyethene, which is the monomer and which is the repeat unit?
- Monomer = ethene
- Repeat unit = polyethene
What happens to ethene molecules during the formation of polyethene?
[3]
(1 + 2-way + 1)
- Double bond breaks
- Electron pair forms new covalent bond
- on a carbon from neighbouring molecule (ethene)
- Forming saturated polymer chain
If u were going to a structural formula of a polymer, what to be aware of?
The C-C inside are actually double bonds
Great job, u know what to do
o7
