Unit 4 - Organic Reactions and Reaction Pathways Flashcards
Effect of Size on reactivity
Small molecules exhibit large changes in character; however, larger molecules will have significant non-polar properties due to the non-polar ‘tail’.
For example: when the molecule is small, the polarity of the OH tail is very influential but as the chain increases, the polar influence of the OH decreases,
Means ethanol is very soluble in water but octanol is less
Example of Polar Functional Group
Another example of a polar functional group is the carboxyl group (-COOH) in carboxylic acids.
Electrophiles
A dipole is created in the region of the carbon atom where oxygen is more electronegative than carbon causing a local area of partial negative charge
Species that have a positive charge or an area of partial positive charge are called electrophiles (‘electron-liking’). They react with electron rich area.
Nucleophiles
Species that have a negative charge or area of partial negative cahrge are called nucleophiles (‘nucleus liking’)
Electrostatic charge interactions between the species form the basis for many organic chemistry reactions
Effects of Double and Triple Bonds
The electrons in the double bond or triple bond of an alkene or alkyne often take part in chemical reactions
The greater the bond energy, the greater the energy required to break the bond
Tests to Detect Classes
carboxylic acid - add water to and test for pH - dissociation of the acid and release H+ into solution
amines - add to water and test for pH - ionisation of the base and release of OH- into solution
alkenes - add bromine water, colour change from brownish-red to colourless indicates an alkene - two bromine toms are added to the molecule at the position of the double bond
1,2,3 alcohols - add acidified potassium dichromate (or potassium permagenate) and heat, a colour change from yellow to green is observed for 1,2 alcoholds - 1 alcohols react to form carboxylic acids, 2 alcohols react to form ketones
Determination of organic acid and bases using a pH indicator
The strong polarity of the carboxylic acid functional group will result in easy dissociation of the molecule in water. H+ is released
Universal indicator, or by adding carbonate, the presence of aqueous H+ can be detected
Determination of organic acid and bases using a pH indicator #2
Amines are organic bases that react with hyrdogen ions and so result in a greater concentration of OH+ ions in the solution
Determination of primary, secondary and tertiary alcohols
1 alcohols will react to form an aldehyde and then be further oxidised to a carboxylic acid (yellow to green)
2 alcohols will react to form a ketone but not futher to a carboxylic acid (yellow to green)
3 alcoholds (yellow - no change)
Combustion of Alkanes
Alkanes react with oxygen to produc CO2 and significant amounts of energy
It is an exothermic redox reaction
If the supply of oxygen is plentiful, the products of combustion will be CO2 and H2O
Substitution Reactions of Alkanes
Sub of alkanes - haloalkanes
The reaction can only occur with UV light and not at room temp, in a dark vessel
———————————— UV light———————
General fromula - RH + Cl2 ————-> RCl + HCI
Substitution Reactions of Haloalkanes
Carbon - halogen bonds are weaker than bonds without a dipole as the bond is polar
Species such as another halogen ion (OH-), a cyanide ion (CN-), ammonia or hydroxide ion (OH-) can be subbed for the halogen in the polar bond.
Substitution Reactions of Haloalkanes #2
Formula for formation of nitrile by substitution with potassium cyanide or sodium cyanide is
—————Heat———–
RCl + CN- ————-> RCN + Cl-
Solution must be anhydrous as the presence of water will allow (OH) to be subtituted rather than (CN)
A new atom is also added, nitrogen, meaning that the molecule now has a key part of the basic structure of an amino acid
Substitution Reactions of Haloalkanes #3
Ammonia also reacts with haloalkanes in a subtitution reaction, the produc of these reactions is an amine
General equation: RCl + NH3 ——-> RNH2 + HCl
Substitution Reactions of Haloalkanes #3
Reacts with sodium hydroxide to form an alcohol
General formula: RCl + NaOH ——-> ROH + NaCl
Conditions of Haloalkane Reactions
When the reaction is with sodium hydroxide or potassium hydroxide, a haloalkane can undergo subtitution reaction
heat is applied - elimination is favoured
higher temps favour substitution (alcohol)
lower temps favour elimination (alkene)
primary - substitution
secondary - both
tertiary - elimination
water - substitution
solvent - elimination
heat - elimination
High NaOH concentration - elimination
Addition reaction of alkenes
Alkenes are more reactive compoiund than alkanes. The reactions of alkenes usually involve the addition of a small molecule to the double bond of alkene
* two reactants comvine to form one product molecule
* the C-C double bond becomes a C-C single bond
* unsaturated becomes saturated
* added acroos the double bond
Example of Alkene Addition Reactions
When ethene reacts with a small molecule, each atom of the molecule forms a covalent bond to a carbon atom in the double bond.
Unlike substitution , there is no inorganic product formed
All atoms the reactants end up in the final product
Reduction of Alkanes with Hydrogen - Reduction of Alkanes
Alkanes react with hydrogen gas in the presence of metal catalyst (nickel) and heat, to form alkanes
unsaturated - saturated
Activation energy is too high for the reactants to proceed at room temperature without a catalyst
Catalysts - metal, palladium and platnimum
Alkynes can be broken down into alkanes by addition of hyrdrogen atoms under greater heat and strong metal catalyst
Reaction of Alkenes with Water
By reacting with water, an alkene can form a H and OH bond in place of the double carbon bond to create alcohol
A phosphiric acid (H3PO4) catalyst and 300 degree celsius temperature is required
Heterogenous - solid/gas
Reaction of Alkenes with Hydrogen Halides
This addition reaction can produce two isomers.
One isomer is produced more than the other.
It can be preducted using Markinovs rule
Markinov’s Rule
States that during the addition of Hx or H2O to an assymetrical alkene, hydrogen adds to the carbon of the double that has the greater number of hydrogens attached.
(2-cholorbutane, 1-chlorobutane)
Addition reactions of Alkenes to form Poly (Alkenes)
Polymers are very long molecules formed from the linking of a large number of small molecules called monomers
Many ethene monomers against a catalyst form to produce a polyalkene
Elimination reaction of Haloalkenes
Haloalkane loses two atoms (one hydrogen and one halogen) and regains a double bond
Saturated - unsaturated
Requires strong base such as sodium hydroxide and heat to proceed
Lower temperatures favour substitution
