Unit 4 - Organic Reactions and Reaction Pathways

Question 1

Effect of Size on reactivity

Answer 1

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

Question 2

Example of Polar Functional Group

Answer 2

Another example of a polar functional group is the carboxyl group (-COOH) in carboxylic acids.

Question 3

Electrophiles

Answer 3

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.

Question 4

Nucleophiles

Answer 4

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

Question 5

Effects of Double and Triple Bonds

Answer 5

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

Question 6

Tests to Detect Classes

Answer 6

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

Question 7

Determination of organic acid and bases using a pH indicator

Answer 7

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

Question 8

Determination of organic acid and bases using a pH indicator #2

Answer 8

Amines are organic bases that react with hyrdogen ions and so result in a greater concentration of OH+ ions in the solution

Question 9

Determination of primary, secondary and tertiary alcohols

Answer 9

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)

Question 10

Combustion of Alkanes

Answer 10

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

Question 11

Substitution Reactions of Alkanes

Answer 11

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

Question 12

Substitution Reactions of Haloalkanes

Answer 12

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.

Question 13

Substitution Reactions of Haloalkanes #2

Answer 13

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

Question 14

Substitution Reactions of Haloalkanes #3

Answer 14

Ammonia also reacts with haloalkanes in a subtitution reaction, the produc of these reactions is an amine
General equation: RCl + NH3 ——-> RNH2 + HCl

Question 15

Substitution Reactions of Haloalkanes #3

Answer 15

Reacts with sodium hydroxide to form an alcohol
General formula: RCl + NaOH ——-> ROH + NaCl

Question 16

Conditions of Haloalkane Reactions

Answer 16

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

Question 17

Addition reaction of alkenes

Answer 17

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

Question 18

Example of Alkene Addition Reactions

Answer 18

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

Question 19

Reduction of Alkanes with Hydrogen - Reduction of Alkanes

Answer 19

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

Question 20

Reaction of Alkenes with Water

Answer 20

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

Question 21

Reaction of Alkenes with Hydrogen Halides

Answer 21

This addition reaction can produce two isomers.
One isomer is produced more than the other.
It can be preducted using Markinovs rule

Question 22

Markinov’s Rule

Answer 22

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)

Question 23

Addition reactions of Alkenes to form Poly (Alkenes)

Answer 23

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

Question 24

Elimination reaction of Haloalkenes

Answer 24

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

Question 25

Oxidation and Reduction of Alcohols and Nitriles

Answer 25

Alcohols an nitriles have functional groups which give their molecules very differnet properties from the alkane or haloalkane molecules
Differences relate to the charge imbalance in the molecules produces by changes to the structure

Question 26

Oxidation and Reduction in Organic Reactions

Answer 26

Oxidation - the gain of the oxygen atom (s) and loss of hydrogen atoms (s)
Reduction - the loss of oxygen atom (s) or the gain of hydrogen atoms (s)

Question 27

Primary, Secondary and Tertiary Alcohols Oxidation

Answer 27

Primary first oxidise to aldehyde - then to carboxyl acid
Secondary oxidise to a ketone
Tertiary do not react

Question 28

Combustion of Alcohol

Answer 28

Produces carbon dioxide, water and significant amounts of energy. Exothermic redox reaction.
Only carbon dioxide and water if it is a complete combustion
Combustion is oxidation
Example of ethanol combustion:
C2H5OH + 3O2 +2CO2 + 3H2O

Question 29

Oxidation of Primary Alcohols

Answer 29

A primary alochol can be oxidised to form carboxylic acid in two stages. It requires an oxidising agent such as acidified potassium dichromate or acidified potassium permanagate.
First stage - primary alcohol is oxidised to aldehyde by removing two hydrogen atoms
Second stage - aldehyde turns to carboxylic acid by addition of an oxygen atom

Question 30

Oxidation of Secondary Alcohols

Answer 30

Hydrogen is removed from OH bond and becomes a =O bond and the carbon atom also loses a hydrogen atom

Question 31

Oxidations of Tertiary Alcohol

Answer 31

Does not occur. The carbon attached to yudroxyl group does not have a hydrogen C-H bond to break so oxidation cannot occur
colour change from orange to green

Question 32

Reduction of Nitriles

Answer 32

Involves adding hydrogen atoms at the position of the triple bond and agrees with our defintion of reduction as ‘adding hydrogen’. Results in C-N single bond and a terminal amino group (NH3)
Increased temperature and metal catalyst such as nickel and palladium is required.
Nitrile - reduction - amine

Question 33

Condensation Reaction

Answer 33

Reactions that involve the combination of two reactants and elimination of small molecules, such as water, are called condensation reactions.
When functional groups on the seperate molecules react, they are covalently bonded.
Creates a structure that has combined size of reactants

Question 34

Esterification

Answer 34

In an ester formation, it is the hydrogen atom from the (OH) of the acohol and the (Oh) from the cabroxylic acid, that combine to form water
Which is the molecule eliminated in this condensation reaction.

Question 35

Hydrolysis

Answer 35

Ester can react with water to form a caboxylic acid and an alcohol
Catalysed by an alkali or dilute acid and required heat to proceed
In the ester, the chain contaning C=O has three carbons and so it is deduced that it was derived from the carboxylic acid
Chain on other side of the oxygen atom has two carbons and so it is derived from ethanol

Question 36

Amide Formation