mechnaisms test revision Flashcards
What is free radical substitution?
The substitution of alkane H atoms with halogen free radicals.
What is a free radical?
An unpaired electron represented by a dot and are highly reactive.
What is initiation?
The formation of free radicals. Addition of UV LIGHT turns Cl2 into Cl * + Cl *.
What causes the production of free radicals?
In the case of Cl2, one electron goes to the Cl atom and the other goes to the other. This is homolytic fission. UV light causes this to happen as it has the correct frequency that breaks the bond between Cl2.
What is propagation?
Chlorine free radical starts a chain reaction. Products are formed but free radical remains.
e.g. reacts with CH4.
What products are made in the propagation stage of free radical substitution?
GH4 + CL* -> *CH3 + HCl (methyl radical).
CH3 + CL2 -> CH3Cl + CL (chloromethane product).
What is termination?
Free radicals combine.
Cl* + Cl* -> Cl2.
CH3 + Cl -> CH3Cl.
*CH3 + *CH3 -> C2H6 (twice the size of the original).
CH4 + Cl2 -> CH3Cl + HCl.
What are the problems with free radical substitution?
Will not occur in the dark. Substitution is random, so there is no control over which H is substituted in larger alkanes. If left to run, multiple Cl gas substitutions will occur. Multiple products are made, so it’s not a precise process.
How do CFCs and UV light interact?
UV light can break C-Cl bonds in CFCs to form chlorine free radicals. The C-F bond is too strong to be broken by UV. The chlorine radicals react with ozone and decompose it:
Cl+ + O3 → ClO+ + O2.
The chlorine oxide radical can react with ozone and decompose it:
ClO* + O3 → 2O2 + *Cl.
A chlorine radical is then produced which can go on to continue this reaction many thousands of times.
What is a nucleophile?
A nucleophile is an electron-rich species that can donate a pair of electrons.
Why is a hydroxide a better nucleophile than water?
Because it has a full formal negative charge, whereas the oxygen atom in water only carries a partial negative charge.
What are the typical nucleophiles?
OH- (hydroxide ion), CN- (cyanide ion), NH3 (ammonia). These all have lone pairs.
What is a nucleophilic substitution reaction?
A nucleophilic substitution reaction is one in which a nucleophile attacks a carbon that has a partial positive charge, replacing an atom that has a partial negative charge.
Why do halogenoalkanes undergo nucleophilic substitution reactions?
Due to the polar C-Halogen bond where there are large differences in electronegativity between the carbon and halogen atom.
What is the general mechanism for nucleophilic substitution?
The nucleophile goes to the carbon atom while the electrons are shifted to the halogen, forming an alkane with the nucleophile on the end and the halogen that is now an ion with lone pairs.
What happens in nucleophilic substitution with cyanide ions?
The ethanolic solution of potassium cyanide is heated under reflux with the halogenoalkane. The product is a nitrile, adding an extra carbon atom to the carbon chain.
What can the reaction of KCl with a halogenoalkane be used for?
Used to make a compound with more than one carbon atom than the best available organic starting material.
What happens in the nucleophilic substitution with ammonia?
An ethanolic solution of excess ammonia is heated under pressure with the halogenoalkane. The product is a primary amine.
What are the conditions needed for substitution reactions?
NaOH (OH-) needs to be aqueous and warm. KCN (CN-) needs to be ethanolic and warm. NH3 needs to be in excess, and concentrated ammonia needs to be dissolved in ethanol at pressure in a sealed container.
How does an elimination reaction occur?
Nucleophiles have lone pairs of electrons, allowing them to form dative covalent bonds with H+; they are bases. Warm, aqueous sodium hydroxide acts as a nucleophile. Nucleophilic substitution reactions occur with halogenoalkanes, forming an alcohol. Under different conditions, OH- will act as a base, removing H+ from the halogenoalkane, resulting in an elimination reaction rather than substitution.
What are the conditions for an elimination reaction?
Heat, ethanol as a solvent, concentrated potassium hydroxide.
What happens in an elimination reaction?
The halogenoalkanes are heated with ethanolic sodium hydroxide, causing the carbon-halogen bond to break heterolytically, forming a halogen ion and leaving an alkene as an organic product.
What is the mechanism for an elimination reaction?
The electrons move to the halogen to form a partially negative charge on the halogen and a partially positive charge on the carbon it is attached to. The nucleophile OH- will take a hydrogen from the carbon, forming an alkene, water, and a halogen ion.
What are the differences in conditions between substitution and elimination?
Elimination has a high temperature; substitution has a low temperature. Elimination has a concentrated amount of hydroxide; substitution is dilute. The solvent in elimination is ethanol; in substitution, it is water. Hydroxide acts as a base in elimination and as a nucleophile in substitution.
