18 KiB
HL Chemistry 3
The course code for this page is SNC4MZ.
Organic chemistry
!!! definition - An organic molecule is one with at least one carbon atom covalently bonded to another carbon or hydrogen atom (i.e., at least one C-H or C to C bond)
Carbon is unique in organic chemistry as it is the only element with the following properties:
- It is in the second row of the periodic table, meaning it has less electron shells, thus forming stronger bonds
- It can covalently bond to up to 4 other atoms
- Because each of its valence electrons is involved in bonding, it can form single through triple bonds
- The molecular geometry can be anything from tetrahedral to linear depending on its bonding
Carbon is also able to bond to itself in the following ways:
- long straight chains
- long straight chains with branches
- rings
(Source:
Kognity)
Simple hydrocarbons
!!! definition - A branched hydrocarbon is one with at least one “side group” extending from the main hydrocarbon chain. - A functional group is a group of atoms responsible for the characteristic properties of a molecule (e.g. C=C) - A homologous series is a family of organic compounds with the same functional group but the hydrocarbon chain length changes by 1 \(\ce{CH2}\) group.
These only contain carbon and hydrogen.
Alkanes are a homologous series that only contain single bonds between carbons, and are named with the number of carbons with the suffix “-ane”.
(Source:
Kognity)
| Carbon atoms | Prefix |
|---|---|
| 1 | Meth |
| 2 | Eth |
| 3 | Prop |
| 4 | But |
| 5 | Pent |
| 6 | Hex |
| 7 | Hept |
| 8 | Oct |
| 9 | Non |
| 10 | Dec |
!!! example A molecule with only hydrogen and three carbon atoms all held together with single covalent bonds is called “propane”.
Alkenes contain at least one carbon-carbon double bond and are named with a prefix with the total number of carbon atoms and “-ene”.
Alkynes contain at least one carbon-carbon triple bond and are named with a prefix with the total number of carbon atoms and “-ene”.
!!! warning The lack of standardisation prior to IUPAC means that some IUPAC names have common names that are still widely used today.
- acetylene: **ethyne**
- vinyl: **propene**
- ethylene: **ethene**The general formula for an acyclic hydrocarbon with no rings is as follows, where \(n\) is the number of carbon atoms, \(x\) is the number of double bonds, and \(y\) is the number of triple bonds. \[\ce{C_nH_{2n+2-2x-4y}}\]
Representing organic compounds
A simple molecular formula is the least useful as it provides no information on structure and bonding. \[\ce{C6H14}\]
A complete structural diagram shows all atoms by their chemical symbols and uses lines like a Lewis Dot diagram to represent bonds. VSEPR shapes do not need to be taken into account because these are 2D representations of the molecule.
A condensed structural diagram is a complete structural diagram but C-H bonds are aggregated into a formula. \[\ce{CH3 - CH2 - CH2 - CH2 - CH2 - CH3}\]
A structural formula or expanded molecular formula is a condensed structural diagram but there are no bond lines. The bond organisation is inferred based on the number of hydrogens on each carbon. Carbon chain side groups (branches) are shown with parentheses. \[\ce{CH3CH2CH2CH2CH2CH3}\]
A condensed structural formula is a structural formula but any consecutive repeated \(\ce{CH2}\) groups are factored with parentheses. \[\ce{CH3(CH2)_4CH3}\]
A line diagram or skeletal structural formula removes carbons and hydrogens and replaces all carbon-carbon bonds with lines, where the number of lines represents the type of bond. Each line is bent where a carbon atom would be, except for triple bonds as those are linear. Non-carbon groups such as \(\ce{OH}\) can be shown in collapsed form.
!!! example These are the ways to represent pentane, \(\ce{C5H12}\). The structural formula is
mislabeled as a condensed structural diagram.
(Source:
Kognity)
General nomenclature
To name an organic compound:
- Find the longest acyclic chain of carbon atoms as the parent chain.
- Assign numbers from 1 to \(n\) for
each carbon atom in the parent chain.
- The numbers should be arranged in a way that the highest priority functional group in the chain is assigned the lowest number possible.
- Apply the first branch rule only if there is a tie:
If there are side chains, the parent chain should be numbered such that
the location of any side chains have the lowest number possible.
- If there is a tie, the location with the most branches wins.
- If there is a tie, the rest of the chain is compared in sequence applying the first branch rule.
- If there is a tie, the first location with the side chain group name that is alphabetically greater wins.
- If there is a tie, it doesn’t matter which side is picked as the whole thing is symmetrical.
- Name the main chain based on the name of the functional group and location number for the functional group in the format “number-name”.
- Name the side groups.
- If the group is not carbon, name it by its identity.
- Otherwise, name the hydrocarbon based on the number of carbons in the side group with the ending “yl”.
- If there is more than one identical side group in the whole chain, combine their numbers and names with a Greek prefix.
- Assign a number representing the carbon atom of the parent chain that the side group is attached to in the form “numbers-name”.
- Arrange the name with each side group with their numbers in alphabetical order, discounting any prefixes due to duplicates, followed by the parent chain.
- Join everything together:
- Drop the ending vowel from the prefix if there is a double vowel unless it is “i”.
- Separate numbers from words with dashes.
- Separate numbers from numbers with commas.
- Do not separate words from words.
!!! tip In hydrocarbons:
- Atoms with double or triple bonds share equal priority as the highest functional group.
- The main chain will be named as an alkane if there are only single bonds.
- If there is exactly one double or triple bond, it will be named as an alkene or alkyne with its position inserted between the prefix and ending.
- e.g., "pentane", "pent-2-ene"
- If there are multiple double or triple bonds, their numbers are also included, but an "a" is appended to the prefix and a Greek prefix added to the suffix.
- e.g., "penta-1,3-diene", "hexa-1,3,5-triyne"
- If there are both double and triple bonds, the "-ene" becomes "-en" and is always before "-yne".
- e.g., "pent-4-en-2-yne"!!! example tf
Other side chains with equal priority as double or triple bonds in side chains include:
- halogens, which have their “-ine” suffix replaced with “o” (e.g., “chloro”)
- \(\ce{NO2}\): “nitro-”
- benzene (as a side chain): “phenyl”
If there is no other option and there is a branched side chain, name it based on the total number of carbon atoms in the side chain.
!!! example tf
Cyclic aliphatic hydrocarbons
These contain rings that are not benzene rings.
\[\ce{C_nH_{2n-2x}}\]
!!! warning Cyclic hydrocarbons do not contain any triple bonds as it would force the carbon ring to widen too much.
Cyclic aliphatic hydrocarbons are named the same way as acyclic hydrocarbons except they have a “cyclo-” at the start of the name of their parent chain.
!!! example cyclohexa-1,3-diene
The initial double bond should be numbered such that the lowest number is assigned to both sides of the bond (numbers 1 and 2 should be to either side of the double bond). If there is more than one double bond, the ring should be numbered such that the lowest number is assigned to both.
The first branch rule still applies. (See HL Chemistry 2#General nomenclature.)
!!! example tf
!!! warning Rings can be side chains, and are named accordingly (e.g., “cyclopropyl”). The “cyclo-” prefix is counted when sorting names alphabetically as it describes the group.
Cyclic aromatic hydrocarbons
These contain benzene rings, which do not actually have single/double bonds as they actually have delocalised pi bonds.
(Source:
Kognity)
As benzene rings do not have double bonds, they are named according to the first branch rule.
Isomers
Structural isomers are two chemicals that have the same chemical formulas but have different structural formulas, resulting in different chemical properties.
Hydrocarbon chain isomers are two chemicals with the same chemical formulas but have different carbon/hydrogen arrangements.
!!! example The following are two hydrocarbon chain isomers (and, by
extension, structural isomers) of \(\ce{C5H12}\).
(Source:
Kognity)
Positional isomers are two chemicals with the same chemical formulas and functional groups but have different structural formulas.
!!! example The following are positional isomers (and, by extension,
structural isomers) of \(\ce{C4H8}\).
(Source:
Kognity)
Functional group isomers are chemicals with the same chemical formulas but different functional groups.
!!! example The following are functional group isomers (and, by
extension, structural isomers) of \(\ce{C3H6O2}\).
(Source:
Kognity)
Geometric or cis/trans isomers are two chemicals have the same chemical formulas and atom arrangements but are positioned differently, thus having ambiguous names.
In order for this to occur, there must be two different atoms or groups of atoms bonded to each carbon atom in the double bond.
- A cis hydrocarbon isomer will have its main chain enter and exit the double bond on the same side.
- A trans hydrocarbon isomer will have its main chain enter and exit the double bond on opposite sides.
Unlike the examples below, these should be named with “cis” or “trans” at the beginning as a separate word without a hyphen.
!!! example The following are two geometric isomers of but-2-ene:
(Source:
Kognity)
- In acyclic compounds, this is because the double bond prevents simply rotating one side but not the other as it would force breaking the pi bond.
- In cyclic compounds, this is because the ring’s other side is similar to a double bond, preventing rotation around the axis.
!!! example The following are cis-trans isomers of
dichlorocyclobutane (notice the chlorine):
(Source:
Kognity)
Isomers may have different physical properties in:
- polarity: a cis isomer may cause a molecule to be polar as opposed to its trans variant
- packing efficiency: a non-branching hydrocarbon chain will pack better than a branching one, and a continuously trans chain will pack better than a cis one
These change the strength and type of intermolecular forces involved so affect their melting/boiling points.
Isomers may also have different chemical properties as cis isomers are more likely to bump into themselves to make some reactions more viable
Benzene reactions
!!! definition - An electrophile is any species that is or would be electron deficient (+) in the presence of a pi bond.
In reactions involving a benzene ring, the ring itself is stable and will not break apart because of the strength of delocalised pi bonds.
Therefore, only the hydrogens can be swapped out via electrophilic substitution, where an hydrogen atom is substituted with an electrophile. The concentration of electrons in the delocalised pi area attracts electrophiles to initiate the bond.
In the mechanism diagram below, \(\ce{E+}\) represents the electrophile. Curly arrows are used to show the movement of electrons from the delocalised area to the electrophile and hydrogen atom to the delocalised area.
(Source:
Kognity)
The first step (the change from the first to the second diagram) is the slow step due to the highest activation energy due to the requirement to break a bond.
(Source:
Kognity)
Benzene nitration
!!! definition - A nitrating mixture is a mixture of concentrated sulfuric and nitric acids.
In a nitrating mixture, benzene will react with positive nitronium ions at ~50°C to form nitrobenzene, outlined in the reaction mechanism diagrams below.
\[\ce{C6H6 + HNO3_{(aq)} ->[conc H2SO4][50^\circ C] C6H5NO2 + H2O_{(l)}}\]
(Source:
Random Quora Person)
The first step is to form the nitronium ion through a Bronsted-Lowry acid-base reaction between the acids.
\[\ce{HNO3_{(aq)} + H2SO4_{(aq)} <=> H2NO3+_{(aq)} + HSO4-_{(aq)}}\]
The lone pair on the oxygen of the nitric acid attracts a hydrogen atom, which becomes an \(\ce{H+}\) ion as sulfuric acid’s oxygen takes its electrons. The hydrogen ion bonds to the nitric acid.
\[\ce{H2NO3+_{(aq)} <=> H2O_{(l)} + NO2+_{(aq)}}\]
The oxygen-hydrogen group is conveniently able to form water by taking both electrons it was sharing with the nitrogen. The other single-bonded oxygen compensates with a dative covalent bond with the nitrogen to form the nitronium ion.
The second step is to react with benzene through electrophilic substitution, with electrons moving back from the dative oxygen-nitrogen bond back to the oxygen.
Alkane reactions
!!! definition - Halogenation is the introduction of a halogen into a compound.
Substitution halogenation
Because a sigma bond must be broken, alkanes are not very reactive. In the presence of light, alkanes will react with halogens in their standard state through halogenation, replacing one of their hydrogens. Fluorine is an exception that does not require light because it is highly reactive.
If the halogen is in excess and the reaction continues, more of the halogen (not the hydrogen-halogen product) will react with the alkane until all hydrogens have been substituted.
!!! example \[\ce{CH3CH3 + Cl2_{(g)} ->[light] CH3CH2Cl + HCl_{(g)}}\]
If $\ce{Cl2}$ is in excess:
$$
\ce{
CH3CH2Cl + Cl2_{(g)} ->[light] CH3CHCl2 + HCl_{(g)} \\
... \\
CCl3CHCl + Cl2_{(g)} ->[light] CCl3CCl3 + HCl_{(g)}
}
$$The order that hydrogens are substituted in is random. If there is more than one possibility, all of them are written as products, ignoring balancing.
!!! example Propane reacts with chlorine gas to form either 1-chloropropane or 2-chloropropane. \[\ce{CH3CH2CH3 + Cl2 ->[hf] CH3CH2CH2Cl + CH3CHClCH3 + HCl}\]
!!! example 1-bromoethane reacts with chlorine gas to form either 1,1-dibromoethane (40% chance) or 1,2-dibromoethane (60% chance) because each hydrogen is equally likely to be substituted, and there are 2 and 3 that would form them, respectively. \[\ce{CH2ClCH3 + Cl2 ->[hf] CHCl2CH3 + CH2ClCH2Cl + HCl}\]
Free radical substitution
!!! definition - A free radical is a species with a lone unpaired electron. - Homolytic fission is the dissociation of a chemical bond in a neutral molecule where each product takes one electron, generating two free radicals. - Heterolytic fission is the dissociation of a chemical bond in a neutral molecule where one product takes both electrons.
The free radicals are first produced with the help of light energy.
\[\ce{Br2 ->[hf] Br. + Br.}\]
They are then spread to organic compounds and reformed.
\[ \ce{ Br. + CH4 -> .CH3 + HBr \\ Br2 + .CH3 -> CH3Br + Br. } \]
This cycle only ends when all radicals are used up, through reactions that end up with a net loss in radicals, such as:
- \(\ce{Br. + Br. -> Br2}\) (unlikely, contributes a little)
- \(\ce{.CH3 + Br. -> CH3Br}\) (likely)
- \(\ce{.CH3 + .CH3 -> CH3CH3}\) (likely)
!!! warning The free radical is on the carbon atom, not the hydrogen atoms, so the marker goes at the beginning.
Alkene/yne reactions
The presence of double/triple bonds make alkenes and alkynes more reactive and also allow the addition of species as pi bonds are easier to break.
Addition halogenation
These spontaneous reactions break the double/triple bond down a level and slot themselves in (i.e., alkynes form alkenes, alkenes form alkanes).
\[ \ce{alkene + Br2 -> alkaneBr2} \\ \ce{alkyne + Br2 -> alkeneBr2} \]
(Source:
Kognity)
!!! warning If an alkene is formed, the same randomness of hydrogen substitution applies, so there is a chance that a cis/trans isomer is be formed.
!!! example This process is used to test for alkenes/alkynes in a solution. As bromine water is red-brown, if alkenes/alkynes are present, the water will be decolourised from red-brown to more colourless.
Hydrogenation
The addition of hydrogen follows the same principle as that of halogenation.
\[\ce{alkene + H2 ->[\text{heat, high pressure, Ni/Pt/Pd}] alkane}\]