Lecture 5 Flashcards
(15 cards)
Classes of alkyl halides
- Alkyl halide: Halogen, X is directly bonded to sp^3 carbon.
- Vinyl halide: X is bonded to sp^2 carbon of alkene.
- Aryl halide: X is bonded to sp^2 carbon on benzene ring
Properties of alkanes
- Physical state
- Melting and Boiling point
- Solubility
- Combustion
- Reactivity
- Substitution reaction
Physical State
- Lower alkanes (methane, ethane, propane, butane) are gasses at room temperature
- Higher alkanes; -up to 17, 18 carbon atoms- liquids
- Even higher alkanes -solid at room temperature.
Alkanes with 1-4 carbon atoms used as fuels
Alkanes with 5-8 carbon atoms are very volatile and are used to make petroleum.
Alkanes with 9-17 carbon atoms are found in kerosene, diesel and jet fuel.
Alkanes with 18 or more carbon atoms are waxy solids and are used in waxy coatings of fruits and vegetables.
Physical properties cont.
Straight chains - easier to pack together, means more opportunity to interact
It also means stranger intermolecular forces = higher b.p.
Melting and boiling point
Boiling point increases as the number of carbons (chain length) increases.
Bigger molecules are stronger: stronger intermolecular forces (increased Van Der Waals interactions)
Boiling points of alkanes depends on more molecular mass (chain length). The more intermolecular mass is added, the higher the boiling point
Solubility
Alkanes are insoluble in water (water is polar) but soluble in organic liquids (non-polar),
It is because ether are made from carbon and hydrogen there is no real difference in electronegativity between them.
Non-polar: less dense than water
Combustion (complete)
-Alkanes are very flammable
-For complete combustion to occur there must be any adequate supply of oxzygen
-Complete combustion leads to the formation of CO2 and H2O
-If combustion is complete, all hydrocarbons will burn in blue flame
Combustion (incomplete)
-Incomplete combustion, occurs with limited O2. CO, carbon (soot) and water are the by products.
-It is recognisable by a flame that burns dirty yellow/orange.
-In general combustion tends to be lass complete ad the number of carbon atoms in a molecule rises.
-Bigger hydrocarbons are more likely to give yellow smoky flame (formation of CO or C)
Combustion
-Hydrocarbons become more difficult to ignite as the molecules get larger - larger molecules don’t vaporise so easily (the reaction is better when the oxygen and hydrocarbon are mixed well together as gasses.)
-Bigger molecules, greater Van der Waals attraction (this makes them more difficult to break away and turn to a gas)
-If combustion complete, all hydrocarbons will burn in blue flame formation of CO2
If incomplete combustion - will burn with yellow/orange
Reactivity
Reactivity for alkanes tends to be low - alkanes are typically inert under normal conditions and tend not to react with alkalis, acids or oxidising and reducing agents.
A resin for low reactivity is the non- polar nature of C-H bonds. However, under suitable conditions they can undergo:
1. Combustion
2. Cracking and Hydro cracking
3. Substitution reactions
Cracking and Hydro cracking
-Cracking of large hydrocarbons at high temperature produces smaller hydrocarbon
-Important process in petroleum refining
-It is done in the presence of hydrogen (hydro cracking) or in the presence of catalyst (catalytic cracking)
Substitution reactions
-Substitution reaction of alkanes occurs= by replacement of hydrogen atoms with more reactive atoms such as chlorine
-Radicals — molecular fragments with an odd number of shared electrons (at least one unpairs electron), very reactive
-Free radicals are like robbers (which are deficient in energy)
-They attack and snatch energy from others to satisfy their octet structure.
Free Radical Mechanism of Halogenation
STEP 1: Initiation.
- Producing a small number of reactive radicals by breaking relatively weak bond Cl-Cl producing Cl radicals. Short lived species and never present in large concentrations.
STEP 2: Propagation.
- Reactive chlorine radicals collide with methane
STEP 3: Termination.
- Two radicals might collide as well and combine to form stable product. The side reactions that consume some of the free radical intermediates without generating new ones will stop or slow down the reaction.
Isomerism in Alkanes
-Molecular formula tells us how many atoms are in the compound but the structural formula tell more than that. It shows how the atoms are arranged (how they are connected).
-Molecules with the same number and type of atoms but different structural arrangements are called isomers.
-Isomers often differ in terms of their physical properties and effect on their environment.
Constitutional Isomers
The same molecular formula
Different names
E.g. C11H24 - can be 4-ethyl-4-methyloctane or 4-ethyl-2-methyloctane
