11-12 Organic Chemistry Flashcards
What is a sigma bond?
Def: Atom orbitals overlap directly between the bonding atoms
- Each overlapping orbital contains one electron, so the sigma bond contains two electrons shared
- 4 sigma bonds
How would you sketch a sigma bond/electron density of a sigma bond?
C 0 C (oval)
What is the shape around a carbon atom and why?
Tetrahedral with 109.5 angles
- 4 bonding pairs (4 sigma bonds)
- Repel as far apart as possible
- Saturated hydrocarbons are always tetrahedral
Why are the 3D shapes of alkanes constantly changing?
The molecules can rotate freely about the sigma bonds
Why do alkanes have different boiling points?
As molecules increase in length:
- There are more electrons
- Greater surface contact between molecules (as molecules have a larger surface area)
- More and stronger London forces
- More energy required to overcome the forces
Why do only London forces act between molecules?
Non-polar so only London forces
What is the trend with boiling point and branching and why?
More branching = Lower boiling point
- Less surface contact is possible
- Weaker and/or fewer London forces
- Less energy required to overcome them
Why do you need to compare isomers when comparing boiling points?
Same number of electrons - branching is the only factor changing
Why do alkanes have low reactivity?
- C-C and C-H sigma bonds are strong
- C-C bonds are non polar
- C and H re so similar in electronegativity that C-H bonds are effectively non-polar
What are the advantages from using alkanes as fuels?
- Give out lots of energy
- Readily available
- Easy to transport
- When burned in plenty of oxygen, no toxic products are formed
What is produced in complete combustion of alkanes?
Carbon dioxide and water
How much extra oxygen is needed for complete combustion as you descend the homologous series?
1 1/2 O2
What are alkanes useful for?
Fuels - main components of natural gas and crude oil
Very stable
What is formed during incomplete combustion (limited supply of oxygen)?
Carbon monoxide (CO) and Carbon (C)
How do alkanes react with halogens?
UV radiation in sunlight provide initial energy for a reaction to take place
CH4 (g) + Br2 (l) –> CH3Br(g) + HBr (g)
Substitution reaction (hydrogen atom has been substituted by a halogen atom)
What is radical substitution?
Mechanism for the bromination of methane
What are the 3 stages of radical substitution?
Initiation
Propagation
Termination
Why is carbon so special?
- Four electrons in outer shell
- Form four covalent bonds
- Single, double, and triple bonds
- Strong bonds to itself, chains and rings of carbon atoms
What does a saturated hydrocarbon mean?
Single bonds only
What does an unsaturated hydrocarbon mean?
Contains carbon-to-carbon multiple bonds
What is a homologous series?
A series of compounds with similar chemical properties whose successive members differ by addition of a -CH2- group
What is the functional group responsible for?
The molecule’s chemical properties
Define aliphatic
Chains (unbranded or branched) or non-aromatic rings
Define alicyclic
Joined in unbranched or branched rings (not benzene ring)
Define aromatic
Soem or all carbon atoms are in a benzene ring
What are the 3 homologous series of aliphatic hydrocarbons?
Alkanes - single bonds
Alkenes - double C-C
Alkynes - triple C-C
How do you name an aliphatic alkane?
- Suffix - ane
- Longest chain of carbon atoms
- Side chains (alkyl groups)
- Numbers to show position of alkyl groups
- Ordered alphabetically
What if there are 2 chains with the same length?
The most branches is the longest chain
How do you name alicyclic alkane?
- Cycloalkane
- Prefix: cyclo
How do you name alkenes?
Same rules but suffix is -ene
Indicate position of double bond
What is the alkene functional group and suffix?
C=C
-ene
Haloalkane functional group
(R is any alkyl group) and prefix
R-X
- Cl -Br -I
X = Cl, Br, I
chloro-, bromo-, iodo-
Alcohol functional group
(R is any alkyl group) and suffix/prefix
-OH
- hydroxy
-ol
Aldehyde functional group
and suffix
C
||
C
/ \
R H
-al
-CHO
(Do not need numbers as group always on 1)
Ketone functional group and suffix
O
||
C
/ \
R R
-one
-C(CO)C-
Carboxylic acid functional group + suffix
O
||
C
/ \
R OH
-oic acid
-COOH
Ester functional group and suffix
-COOC-
-oate
Acyl chloride functional group and suffix
-COCl-
-oyl chloride
Amine functional group, prefix and suffix
-NH2
amino-
-amine
Nitrile functional group and suffix
-CN
-nitrile
What is the molecular formula?
Number and type of atoms of each element present in a molecule
What is the empirical formula?
Simplest whole-number ratio of the atoms of each element present in a compound
What is the general formula?
Simplest algebraic formula for any member of a homologous series
What are the general formulas of:
Alkanes
Alkenes
Alcohols
Carboxylic acids
Ketones
CnH2n+2
CnH2n
CnH2n+1OH
CnH2nO2
CnH2nO
What is the displayed formula?
Shows the relative positioning of all the atoms in a molecule and the bonds between them
What is structural formula?
Smallest amount of detail necessary to unambiguously show the arrangement of the atoms in a molecule
Shows which groups are bonded together
What are the rules of skeletal formula?
- A line represents a single bond
- An intersection of two lines represents a carbon atom
- The end of a line represents a -CH3 group
- Show functional groups (e.g. halogen)
What is a structural isomer?
Compounds with the same molecular formula but different structural formulae
What is homolytic fission?
One electron goes to each atoms (electrons split)
Form reactive radicals (atoms or groups of atoms with unpaired electrons)
What is heterloytic fission?
Both electrons go to one atom
Results in ions
What is does a curly arrow represent?
The movement of electron pairs when bonds are being broken or made
What happens in an addition reaction?
Two reactants join together to form one product
A molecule is added to an unsaturated compound, breaking the double bond and forming a single saturated compound
What happens in a substitution reaction?
An atom or group of atoms is replaced by a different atom or group of atoms
What is an elimination reaction?
Opposite of addition
A small molecule is removed from a larger one
One compound forms two, one of which is unsaturated
What happens in each stage of radical substitution?
Initiation:
Homolytic fission
Stable molecule –> 2 radicals
Propagation:
2 propagation steps, chain reaction
1 radical + 1 stable molecule –> 1 radical + 1 stable molecule
Termination
Both radicals removed from the reaction mixture, stopping the reaction
2 radicals –> 1 stable molecule
Give the radical substitution stages for methane and bromine
1 Initiation:
Br2 –> Br* + Br*
2 Propagation
Br* + CH4 –> HBr + CH3*
3 Termination
Br* + Br* –> Br2
CH3* + Br* –> CH3Br
CH3* + CH3* –> C2H6
How do you get the overall equation for the reaction?
Cancel out the radicals of the propagation steps
What is multiple substitution and use the bromine and methane reaction as an example?
The CH3Br produced can asp react with a bromine radical and a further hydrogen atom is substituted
\+Br2 CH4 --> CH3Br---->CH2Br2...
-HBr
