Organic - Halogenoalkanes Flashcards
What are halogenalkanes?
Not many halogenalkanes occur naturally, but they are the basis of many synthetic compounds. Some examples of these are PVC (used to make drainpipes). Teflon (the non-stick coating on pans), and a number of anaesthetics and solvents. Halogenalkanes have an alkane skeleton with one or more halogen (fluorine, chlorine, bromine, or iodine) atoms in place of hydrogen atoms.
What is the general formula for halogenalkanes?
The general formula of a halogenalkane with a single halogen atom is CnH2n+1X where X is the halogen. This is often shortened to R-X.
How do you name halogenalkanes?
- The prefixes fluoro-, chloro-, bromo-, and iodo- tell us which halogen is present.
- Number are used, if needed, to show on which carbon the halogen is bonded.
- The prefixed di-, tri-, tetra-, and so on, are used to show how many atoms of each halogen are present.
- When a compound contains different halogens, they are listed in alphabetical order, NOT in the order of the number of the carbon atoms to which they are bonded.
What is the nature of the halogenalkane bond?
Halogenalkanes have a C-X bond. This bond is polar, C(delta)+X(delta)-, because halogens are more electronegative than carbon. This means electron density is drawn towards the halogen forming delta positive and delta negative regions. As you go down group 7, the bonds get less polar.
Describe the solubility properties of halogenalkanes.
- The polar C(delta)+X(delta)- bonds are not polar enough to make the halogenalkanes soluble in water.
- The main intermolecular forces of attraction are dipole-dipole attractions and van der Waal forces.
- Halogenalkanes mix with hydrocarbons so they can be used as dry-cleaning fluids and to remove oily stains. (Oil is a mixture of hydrocarbons).
Describe the boiling point properties of halogenalkanes.
The boiling point depends on the number of carbon atoms and halogen atoms.
- Boiling point increases with increased chain length.
- Boiling point increases going down the halogen group (more electrons so stronger van der Waals forces).
How are the effects of the boiling point properties of haloalkanes caused?
Both effects are caused by increased van der Waals forces because the larger the molecules, the greater the number of electrons, and therefore the larger the van der Waals forces.
As in other homologous series, increased branching of the carbon chain will tend to lower the melting point.
Halogenalkanes have higher boiling points than alkanes with similar chain lengths because they have higher relative molecular masses and they are more polar.
What factors determine how readily the C-X bond reacts?
When halogenalkanes react, it is almost always the C-X bond that breaks. The two factors that determine how readily the C-X bond reacts are:
- the C(delta)+X(delta)- bond polarity
- the C-X bond enthalpy
How does bond polarity affect the reactivity of the haloalkanes?
The halogens are more electronegative than carbon, so the bond polarity will be C(delta)+X(delta)-. This means that the carbon bonded to the halogen has a partial positive charge - it is electron deficient. This means that it can be attacked by reagents that are electron rich or have electron-rich areas. These are called nucleophiles. A nucleophile is an electron pair donor.
The polarity of the C-X bond would predict that the C-F bond would be the most reactive. It is the most polar, so the C(delta)+ has the most positive charge and is therefore most easily attacked by a nucleophile.This argument would make the C-I bond least reactive because it is the least polar.
How does bond enthalpy affect the reactivity of the haloalkanes?
The C-X bond enthalpies get weaker going down the group. Fluorine is the smallest atom of the halogens and the shared electrons in the C-F bond are strongly attracted to the fluorine nucleus. This makes a strong bond. Going down the group, the shared electrons in the C-X bond gets further and further away from the halogen nucleus, so the bond becomes weaker.
The bond enthalpies would predict that iodo-compounds, with the weakest bonds, are the most reactive, and fluoro-compounds, with the strongest bonds, are the least reactive.
Which factor is most important in terms of affecting reactivity of haloalkanes?
Experiments confirm that reactivity increases going down the group. This means that bond enthalpy is a more important factor than bond polarity.
What are nucleophiles?
Nucleophiles are reagents that attack and form bonds with positively or partially positively charged carbon atoms. They donate a pair of electrons to form a covalent bond. Halogenalkanes are susceptible to attack by nucleophiles.
A nucleophile molecule is attracted to the positive nuclei (electron deficient areas), so they tend to be negatively charged. Molecules with lone pairs are also nucleophilic.
What are the features of a nucleophile?
- A nucleophile is either a negatively charged ion or has an atom with a delta negative charge.
- A nucleophile has a lone (unshared) pair of electrons which it can use to form a covalent bond.
- The lone pair is situated on an electronegative atom.
Why are halogenalkanes susceptible to attack by nucleophiles?
Because the halogen atom is more electronegative than carbon atoms and so the carbon of the carbon-halogen bond is delta positive.
What is a nucleophile in organic chemistry?
In organic chemistry, a nucleophile is a species that has a lone pair of electrons with which it can form a bond by donating its electrons to an electron deficient carbon atom.
What are some common nucleophiles?
\:OH- = the hydroxide ion \:CN- = the cyanide ion \:NH3 = ammonia
What is a nucleophilic substitution?
In a nucleophilic substitution, the halogen atom is replaced by another atom/group (nucleophile). They all follow essentially the same reaction mechanism.
What is a reaction mechanism?
A reaction mechanism describes the route from reactants to products via a series of theoretical steps. These may involve short-lived intermediates.
What is the rate of reaction affected by?
The rate of reaction is partly affected by the strength of the carbon-halogen bond. The longer the bond, the weaker the bond, the more easily it breaks and the faster the reaction. Therefore, in terms of rate, C-I > C-Br > C-Cl > C-F. Fluoro-compounds are unreactive due to the strength of the C-F bond.
What do curly arrows indicate?
The movement of an electron pair in organic reactions. Curly arrows never move from an area of low electron density. Areas of high/low electron density are likely starting points for reactions.
