Chapter 10: Organics (part 2 - functional group chemistry) Flashcards
What are the properties of alkanes?
- non-polar molcules
- = insoluble in water
- with only london forces between molecules
- means they are volatile (evaporate easily)
- the first 4 members are gases at room temperature
From a global perspective, what is the most important reaction of alkanes?
combustion
e.g. petrol and natural gas
alkane compounds form the bases of fuels
What does complete combustion of hydrocarbons require and produce?
- requires the presence of excess air/oxygen
- produces carbon dioxide and water
When does incomplete combustion occur and what does it produce?
- occurs when there is a limited supply of air/oxygen
- produces carbon monoxide and soot (C) as well as water
What is the actual equation for the combustion of ethane?
the combination of these three equations (from both complete and incomplete combustion)
What is produced if there is not sufficient oxygen present for complete combustion?
CO2, CO and carbon
How is incomplete combustion dirtier than complete combustion?
incomplete combustion produces a smoky flame and the toxic gas carbon monoxide (which binds to hemoglobin more strongly than oxygen does, so oxygen is prevented from being transported by the blood)
How is the reactivity of alkanes?
apart from combustion, alkanes are generally fairly unreactive
Why are alkanes generally fairly unreactive?
- the high strengths of C-C and C-H bonds means that it is generally energetically unfavourable to break them in a reaction
- C-C and C-H bonds are essentially non-polar and so are unlikely to attract polar molecules or ions
In what conditions do alkanes react with halogens?
in the presence of sunlight or ultraviolet (UV) light
there is no reaction in the dark at room temperature
needed because alkanes are unreactive
What is the equation for the reaction between methane and chlorine in the presence of UV light?
What is the equation for the reaction between ethane and bromine (in temrs of full structural formulas as well)?
What is chloromethane an important contributor to?
to the destruction of the ozone layer
What is a significant source of chloromethane from?
from natural sources such as wood-rotting fungi
What is a substitution reaction?
an atom or a group is replaced by a different atom or group
What reaction is occuring?
a substitution reaction
a hydrogen atom in ethane is replaced by a bromine atom
What is mono-substitution?
the replacement of one hydrogen atom in the molecule by one halogen atom
What is free radical substitution?
a hydrogen atom gets substituted by a halogen (chlorine/bromine)
What are the 3 stages of free radical substitution?
Initiation
Propagation
Termination
What happens in initiation of free radical substitution?
Consider the reaction between methane and chlorine
- breaking apart chlorine molecules into separate chlorine atoms
- Energy needed is provided by the UV light
- The Cl-Cl bond (242kjmol-1) is weaker than the C-H bond (412Kjmole-1) and therefore it is the Cl-Cl bond that is broken
- Cl-Cl bond is broken through a process called homolytic fission (covalent bond breaks, one electorn goes back to each atom that made up the original molecule to form two species that are the same)
- The chlorine atoms produced each have an unpaired electron and is represented by the dot
- they are described as free radicals
- Now there is an increase in the number of free radicals
What is the process of homolytic fission?
when the covalent bond (made up of two electrons) breaks, one electron goes back to each atom that made up the original molecule - in this case to form two species that are the same (‘homo’ means ‘same’)
Which stage does homolytic fission occur in the free radical substitution?
initiation
What are free radicals?
are species (atoms or groups of atoms) with an unpaired electron.
Are very reactive because of this unpaired electron
Why are free radicals very reactive?
because they have an unpaired electron