4.1.2 - Alkanes Flashcards
(14 cards)
1
Q
alkanes
A
saturated hydrocarbons containing single C–C and C–H bonds as σ-bonds
2
Q
general formula of alkanes
A
CnH2n+2
3
Q
sigma bond (σ-bond)
A
overlap of orbitals directly between bonding atoms, has free rotation
4
Q
shape of alkanes
A
tetrahedral, 109.5° (4 bonding pairs repel each other)
5
Q
how do the BPs of alkanes vary with chain length?
A
- increased chain length
- greater surface area of contact
- more/stronger London forces
- need more energy to overcome
- increased BP
6
Q
how do the BPs of alkanes vary with branching?
A
- more branching
- smaller surface area of contact
- fewer/weaker London forces
- need less energy to overcome
- decreased BP
7
Q
what are the reasons for the low reactivity of alkanes?
A
- high bond enthalpy (C-C and C-H σ-bonds are strong)
- very low polarity of σ-bonds present (C-C bonds are non-polar and C-H bond can be considered non-polar)
8
Q
complete combustion of alkanes
A
alkane + oxygen → carbon dioxide + water
9
Q
incomplete combustion of alkanes
A
alkane + limited oxygen → carbon OR carbon monoxide + water
carbon monoxide (CO) is highly toxic
10
Q
reactions of alkanes with halogens
A
- free radical substitution
- requires UV
- initiation, propagation, termination
11
Q
initiation
A
- covalent bond in halogen broken by homolytic fission
- forms 2 reactive halogen radicals
- energy for fission comes from UV
12
Q
propagation
A
- step 1: halogen radical reacts with alkane to form hydrogen halide and different radical
- step 2: new radical reacts with halogen molecule to form organic product and halogen radical
- chain reaction continues
13
Q
termination
A
- two radicals collide
- forms molecules with all electrons paired
- stops the reaction
14
Q
what are the limitations of free radical substitution?
A
- further substitution
- substitution at different positions in a carbon chain
