Alkanes Flashcards
What type of bonding occurs in alkanes?
- Sigma bonds - end to end overlap of orbitals
What shape is around each c atom in alkanes?
Tetrahedral, 109.5, 4 bonding pairs with each pair of electrons repelling equally
In straight chain alkanes what is the trend in boiling point and why ?
- Boiling point increases as chain length increases
- why? Induced dipole-dipole interactions get stronger as no. of electrons increases
Describe the trend in boiling point in branched chain alkanes + why?
-boiling point decreases as number of branches increases
- why? Less surface contact between chains so IDDs get weaker and need less energy to break
Describe reactivity in alkanes
- unreactive
- high bond enthalpies in molecules
- low bond polarity in sigma bonds. C and H have similar electronegativities
Give the reaction for complete combustion of propane
C3H8 + 5O2 —> 3CO2 + 4H2O
Give the products of the incomplete combustion of an alkane and describe the problems of one product
CO/C and H2O
CO - toxic, reduces ability to carry O2
C(s) - soot can irritate respiratory system
What mechanism is used when reacting alkanes with halogens ?
Radical substitution
What needs to be present for radical sub to be able to take place?
Uv light
Describe the stages of radical substitution
(Cl)
Initiation - uv light breaks halogens covenant bond producing 2 chlorine radicals (homolytic fission)
Cl2 —> 2Cl*
Propagation - radical Chlorine attaches to a H from alkane chain and produces HCl
C2H6 + Cl* —> c2H5* + HCl
Radical alkane reacts with Chlorine to form haloalkane
C2H5* + Cl2 —> C2H5Cl + Cl*
Termination - two radicals combine
Cl* + Cl* —> Cl2
C2H5* + C2H5* —> C4H10
C2H5* + Cl* —> C2H5Cl
What would happen in radical sub if halogen was in excess?
Can end up with Dihalogen substance
