Organic chemistry part 3

Hi guys, you're welcome to mystudyrom my name is David.

Hope both the first and second part helped you a lot and if you are not getting it or feel confused you can use the comment box below….thanks

Today we will be moving to the next part in the organic chemistry so feel relaxed and let's ride on….

Now let's continue from where we stopped in our chemistry. We are going into the alkyne family today.

 The alkyne family:

The alkyne family are unsaturated hydrocarbons with triple bonds attached to one of the carbons present in the compound.

The alkene family has a general formula of CnH_2n-2

So now let's check out the list of compounds present in the alkyne family.

Starting with Ethyne the first of the family.

For the newbies i guess you will be thinking why is it ethyne not methyne?   

I've explained this in the previous part and I'm still going to explain it again now maybe you can get it now.

The alkyne starts with ethyne because if we use their general formula which is CnH2n-2 we won't even get an hydrocarbon, we will only get carbon because hydrogen here will be zero.

Using their general formula CnH_2n-2

Where n=1

C1H_2*1-2

C₁H_2-2

And that is CH₀

And this means C (carbon only) with the absence of H(hydrogen) because hydrogen is now zero.

This is a clear and simple explanation why we don't have methyne. Hope you're with me? Let's ride on…..

So now the first compound ethyne

Using their general formula CnH_2n-2

Where n=2

C₂H_(2*2)-2

C₂H_4-2

C2H2

Ethyne

See the structure below'

Another example is propyne

Using their general formula CnH_2n-2

Prop C=3 (three carbons is prop)

Where n=3

Then C₃H_(3*2)-2

C3H_6-2

C3H4

propyne

See the structure below

Guys i hope you can solve for the rest using that same method because that's the easiest method you can use.

So let's check out the characteristics of the alkyne family.

    Characteristics of the alkyne Family

• They are unsaturated hydrocarbons

• They consists of triple bonds attached to their chains

•   They undergo addition reaction because of the incomplete number of hydrogen bond attached to the carbons.

So now let's see how we can prepare alkynes

Preparation of alkynes:

Lets prepare ethyne as an example

From  calcium carbide Ca₂C

Calcium Carbide reacts with with water to give:

CaC₂ +  H₂O ------>  C₂H₂  +   Ca(OH)₂

Another method of preparing alkynes are:

Dehydrohalogenation of dihalides:

Using KOH  as catalyst, dibromoethene is Dehydrohalogenated as shown below:

Now lets move into the properties of alkynes

Physical properties of Alkynes

1. The first three members of the alkyne family are gases (ethyne, propyne butyne)and while pentyne to Decyne are liquid and the rest solids.

2.  Alkynes have low melting and boiling points.

Their melting point increases as the number of carbon atoms increases.

3.  Alkynes are insoluble in water but are soluble in organic solvents such as benzene.

4.  Alkynes are lighter than water. Their densities increase regularly with increase in molecular mass.

Chemical properties of alkynes

Hydrogenation:

From this word "hydrogenation" we can easily guess that it involves hydrogen.

Hydrogenation is the addition of hydrogen to an unsaturated compound.

Alkyne undergo hydrogenation to form alkanes, this reaction is also known as an addition reaction. This reaction occurs with the use of Kmno4 potassium per magnate.

See the reaction below.

Let's use ethyne as an example :

C₂H₂ +2H₂ ---------> C₂H₆

ethyne   hydrogen      ethane

Two out of the triple bond are eliminated.

Why? are you not thinking so?

The reason is simple. Its simply because these reaction is an addition reaction.

In addition reaction unsaturated compounds are converted to saturated compounds which simply implies that double or triple bonds are removed to give a compound with a single bond.

Check the mechanism in the diagram below:

Halogenation

From the word "halogen" in halogenation we can deduce that it involves the use of the group vii elements which are fluorine, chlorine, bromine and iodine.

Halogen reacts with alkyne to form chloroalkanes. To avoid this long story let's see a reaction on it:

I'm using thesame mechanism I used above:

 Hydration:

Hydration is the addition of water to a compound.

So when we add or let me say hydrate an alkyne we get vinyl alcohol.

 Vinyl alcohol is unstable so of can be re written( this is its isomer) as :

They have thesame molecular formula but different structural formula.

Trimerization:

Tri  means three so from the word trimerization we can guess that we want to triple a compound but how?

Let me use ethyne as an example, if we trimerize ethyne we get benzene :

See the reaction in the diagram below:

Question of the day

• if alkyne undergo addition reaction what will be the final product produced?

• can alkyne undergo substitution reaction?

Post your answers in the comment section below and you are also free to ask questions just put your questions in the comment box.

Guys hope it helped.…….. please comment for any question, guide or help in the comment box below….

see you all in my next post guys…

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Thanks

Holuwarseun…