4.1.2 Alkanes Flashcards
what are alkanes the main components of and why is this the case
- natural gas and crude oil
- stability and lack of reactivity means that deposits of crude oil can remain on earth for years
what types of molecules are alkanes
1) hydrocarbons (ONLY C and H)
2) saturated (ONLY C-C)
what is the general formula of alkanes
CnH2n+2
what bonds the carbons in an alkane
-sigma bonds (σ bonds)
- each carbon is joined to 4 other atoms by single covalent sigma bonds
what is a sigma bond
- the result of the overlap of 2 orbitals, each with 1 electron each, positioned in line directly between the bonding atoms
- act as an axes around which the atoms can rotate freely
- form an electron dense region where - bond is
what shape forms around each carbon in an alkane
- a tetrahedral shape
- 4 e- pairs in the 4 sigma bonds
- pairs repel as far as possible
- bond angles of 109.5 degrees
why is the shape of alkanes not rigid
- the sigma bonds act as an axes around which the atoms can rotate freely (butane can be zig-zag or U shape
why is fractional distillation possible in alkanes
- their boiling points vary as you increase the chain length
what happens as chain length increases in alkanes in its effect on boiling points
1) more e-
2) larger SA so more SA points of contact between molecules
3) more london dispersion forces
4) require more energy to overcome greater forces
- boiling point INCREASES as chain length does
what effect does branching have on alkane boiling points
- as you increase branching:
1) fewer surface area points of contact
2) weaker london dispersion forces
3) branches get in way and stop molecules from getting too close too, decreasing forces more
-boiling point DECREASES as branching increases, (even though molar mass stays the same)
why are alkanes not very reactive
1) the C-C and C-H sigma bonds are strong
2) C-C bonds are non-polar
3) C-H have similar EN, so can be considered non-polar as well
why is the COMPLETE combustion of alkanes useful
- the hydrocarbons are readily available
- hydrocarbons are easy to transport
- no toxic products produced
under what conditions does complete combustion take place
- with plentiful O2
what is the equation for complete combustion of alkanes
CH4 + 2O2 ===> CO2 + 2H2O
- balance in order of C, H, O
- if uneven number of oxygens ever needed, can balance by using fractions
under what conditions does oxygen undergo incomplete combustion
- not enough O2 present
what happens during incomplete combustion and what products does this form
- C not fully oxidised ( H always is to produce water)
- forms CO, toxic gas
- even worse, can form C, solid soot
when do halogens react with alkanes
- in the presence of UV sunlight
- called photolysis (bond breaking using UV light)
what is the mechanism called in which halogens react with alkanes
- radical substitution
what are the 3 steps of radical substitution, and explain what happens in each
1) INITIATION: the halogen molecule breaks into 2 radicals, with UV light ( a form of homolytic fission)
2) PROPOGATION: the halogen radical first reacts with the alkane molecule, and then the alkane radical formed reacts with the halogen molecule
3) TERMINATION: all of the radicals combine ( form desired product and others)
what are the problems with radical substitution
- further substitution with halogen radicals can continue to take place, displacing more halogens past the product (chloroethane turns into dichloroethane)
