Alkanes ✅ Flashcards
What is a sigma bond
A type of covalent bond which forms when atomic orbitals overlap head on, they can rotate freely
What is the bond angle of alkanes
Tetrahedral so 109.5
Why do boiling points of alkenes increase when the number of carbons increases
As chain length increases the molecules have a larger SA, so more contact is possible between molecules. London forces between molecules are greater and so more energy required to overcome the forces
Why do branched isomers have a lower boiling point then a straight isomer
Fewer surface points of contact between molecules for branched, giving fewer London forces. Branches get in the way and prevent branched molecules getting as close together as straight chain molecules, decreasing intermolecular forces further
Kerosene is obtained from crude oil, name the process used and explain how it works (2 marks)
Fractional distillation, different fractions in crude oil have different boiling points. Increase the temp of the system so each fraction vaporizes separately from the other fractions and can be collected
Explain why straight chain isomer of C10H22 has higher BP than any of its branched isomers (2 marks)
More surface contact between straight chain isomer so has more/stronger london forces between chains. More energy required to break these London forces
Explain why BP increases down alkane homologous series (2 marks)
More points of contact between molecules and stronger london forces, more energy required to break these London forces
Describe the bonding in ethane, state and explain the H-C-H bond angle in a molecule of ethane (4 marks)
Single covalent bond between C-C and then C-H bonds are sigma bonds. Sigma bonds is the overlap of orbitals, one from each bonding atom. Each C atom is bonded to 4 other atoms by covalent bonds. Shape is tetrahedral and give a 109.5 bond angle
Why do alkanes not react with most common agents, what reasons are there for lack of reactivity
C-C and C-H sigma bonds are strong
C-C bonds non polar
Electronegativity of C and H so similar that C-H bonds can be considered non polar
Alkanes react with halogens, what is needed for the initial energy for the reaction, what is the name of the reaction
UV light, substitution reaction
What are the 3 stages of a mechanism of alkanes
Initiation, propagation, termination
What happens in the initiation stage of the mechanism
Coolant bond in a bromine molecule broken by homolytic fission, to form 2 highly reactive bromine radicals, energy provided by UV
What happens in stage 2, the propagation stage in a mechanism
Propagation step 1: bromine radical reacts with C-H bond to form a radical of the alkane and hydrogen bromide
Stage 2: radical alkane reacts with bromine molecule forming organic produce
What happens in stage 3, the termination stage
2 radicals collide forming a molecule
What are limitations of radical substitution
Limit its importance of synthesis of just 1 organic compound
Further substitution could occur
Substitution at different points in a carbon chain
