Chapter 12 - Alkanes Flashcards
Alkanes can be?
straight chained, branched, or cyclic
each C atom has a ?
tetrahedral shape - 109.5 bond angle
intermolecular forces?
- C (2.1) & H (2.5) have very similar electronegativity values
- meaning molecules are not polar, weak induced dipole forces form
Reactivity of alkanes?
- have a relatively low reactivity
- the C-H bond has low polarity - the sigma bonds av a low polarity bc the electroneg of C &H is very similar
- the bonds within have very large bond enthalpies - lots of energy needed to break
combustion?
- O2 from air is combined with alkane
- rapid process - (alkenes burn very fast though)
- so alkanes are freq used as, or as a component of a fuel source
- thru combustion, the chemical energy is transferred to thermal energy
Complete combustion of alkanes?
- in excess air (O2)
- max amount of energy transferred
- burns with a clean blue flame
incomplete combustion of alkanes?
- in limited O2 supply
- burns with a dirty yellow flame
products of complete combustion?
- water: simply adds to existing water supplies
- CO2 - greenhouse gas- increasing levels of CO2 are thought to be responsible for increasing global temps
products of incomplete combustion?
- carbon particulates - solid C: observed as soot
- CO - colourless, odourless gas ‘the silent killer’ prevent the effective transport of O2 around the body -> binds to Hb instead of O2
- unburned hydrocarbons - some exist in the products
impurities in the alkanes - sulfur?
- Oxides of S may be produced if S atoms contained within hydrocarbon
- processing such as fractional distillation and cracking may not have removed these impurities
- Sulfur dioxide and Sulfur trioxide may also be formed
- these gases r acidic oxides, when they dissolve in H2O , they form sulfuric and sulfurous acid -> these contriute to acid rain
impurities in the alkanes - N?
- combustion of N containing alkanes will lead to formation of N oxides ➡N monoxide and N dioxide - can dissolve in H20 to form acid rain
catalytic converters?
- are fitted to the exhausts of Road Vehicles
- uses precious metals such as platinum, rhodium, and palladium which are thinly spread over a mesh
- gas pass over the catalyst (e.g. CO, N oxides) ➡ converted into less harmful gases
examples of some of the reactions that occur in the catalytic converter?
-
examples of some of the reactions that occur in the catalytic converter?
- 2CO + O2 ➡ 2CO2
- 2NO + 2CO ➡ N2 + 2CO2
Radical substitution ?
- alkanes react with the halogens, spec. Cl and Br in UV light
what happens in initiation of Radical Sub?
- the radicals are formed
what happens in termination of Radical Sub?
- 2 radicals collide to form a stable product
- (3 equations, this is step 3)
further substituted reactions also occur forming?
more substituted alkanes + longer alkanes
methane molecular formula?
CH4
empirical: CH4
ethane molecular formula?
C2H6
empirical: CH3
Propane molecular formula?
C3H8
empirical: C3H8
butane molecular formula?
C4H10
empirical: C2H5
Pentane molecular formula?
C5H12
empirical: C5H12
Hexane molecular formula?
C6H14
empirical: C3H7
Heptane molecular formula?
C7H16
empirical: C7H16
Octane molecular formula?
C8H18
empirical: C4H9
Nonane molecular formula?
C9H20
empirical: C9H20
Decane molecular formula?
C10H22
C5H11
explain what happens when an alkane boils?
induced dipole forces between mols are broke. NO COVALENT BONDS ARE BROKEN
Explain differences in BP?
- Highest BP = the molecule that has the greatest SA as there is the most induced dipole forces
- lowest= lowest SA so least no. of induced dipole forces between molecules so less energy required to boil
What are aliphatic alkanes and what is their general formula?
- straight or branched chains of C atoms
- GF: CnH2n+2
what r alicyclic/ cycloalkanes + their GF?
Homologous series of alkanes that form rings
CnH2n (same as alkenes)
for a cycloalkane add what to the alkane ?
- just cyclo to start
e. g. cyclohexane
which cycloalkane does not exist?
cycloethane
why are alkanes so unreactive?
- there is only a small difference in electronegativity between C&H meaning molecules not polar, permanent dipole forces cannot arise
- large bond enthalpies, so lots of energy required for a reaction to occur
initiation step definition?
the step that starts the reaction forming radicals by homolytic fission
mechanism definition?
a series of steps that show how the reaction takes place
subsitution reaction definition?
a reaction where an atom of one molecule is swapped for an atom of an other
photochemical reaction definition?
absorption of light energy causing a bond to break
radical or free radical definition?
an atom or group of atoms with an unpaired electron
termination step definition?
radicals react with each other so the reaction stops
homolytic fission definition?
when a bond breaks to give 2 free radicals
series of steps in FRS?
Initiation -> propagation -> termination
- always label
how many initiation steps r there?
1
how many propagation steps r there?
2
how many termination steps r there?
3
conditions for FRS?
UV light
what happens in the initiation step?
- this starts the reaction, forming radicals
- UV light breaks the bond between the 2 Cl atoms homolytically
- this means that each Cl takes one electron back from the bond
- the • represents an unpaired electron
what happens in the propagation steps?
these 2 steps use up the r and make up the p. Hydrogen radicals are very unstable so never form.
- radical + reactant -> product and radical (1)
- radical (1) + reactant -> product + radical
what happens in the termination steps?
radicals react with each other and this ends the reaction. (think of the combinations)
further substitution can occur
(when more of the halogen is added)
what are alkanes?
main components of natural gas and crude oil, are amongst the most stable organic compounds, mainly used as fuels
A sigma bond is?
- the result of the direct overlap of 2 orbitals, one from each bonding atom
- a sigma bond is positioned on a line directly between the bonding atoms
effect of chain length on BP?
- As the chain length increases, the molecules have a larger SA so more surface contact is possible between molecules
- induced dipole forces will be grater and so more energy is required to overcome the forces
branched isomers have a ? BP than straight chain isomers?
lower
why do branched isomers have a lower BP?
- There are less surface points of contact between molecules of the branched alkanes, giving fewer induced dipole forces
- also: shape - the branches get in the way and prevent the branched molecules from getting as close together as straight chain mols, dec. the IMF further
why are alkanes used as fuels?
they r readily available, easy to transport, burn in plentiful supply of O2 w/o releasing toxic products
termination: what happens when 2 radicals collide and react?
the radicals are removed from the reaction mixture, stopping the reaction
further sub can continue until?
all H atoms have been substituted
