Chapter 14.6 - Structure from names of Alkenes and Alkynes

In the previous section, we saw the naming procedure for unsaturated hydrocarbons. In this section, we will see a reverse process. That is., we will be given the IUPAC name of an alkene or an alkyne. We must draw it's structure.

We will learn the process with the help of an example:
Given: IUPAC name is hex-3-ene
We will write the procedure in steps:
1. The word root is hex. So there will be 6 carbon atoms. 
• Draw these 6 carbon atoms with out any bonds between them. 
• Number the carbon atoms from left to right
• This is shown in fig.14.50(a) below:

Fig.14.50

2. The suffix is ene. So there will be a double bond.
• The position of the bond is given as 3 
• This indicates that, the bond is in between carbon atoms 3 and 4
    ♦ The bond cannot be in between carbon atoms 2 and 3
    ♦ This is because, the IUPAC name would then be hex-2-ene
• So draw a double bond between carbon atoms 3 and 4
    ♦ All other carbon-carbon bonds can be shown as single bonds
• This is shown in fig.14.50(b)
3. Fill all the valencies of carbon atoms. 
• Since it is a hydrocarbon, only carbon and hydrogen will be present. 
    ♦ So we use hydrogen to fill up the valencies. 
• The result is shown in fig.14.50(c) above. So this fig.14.50(c) shows the required structure

Another example:
Given: IUPAC name is but-1-yne
We will write the procedure in steps:
1. The word root is but. So there will be 4 carbon atoms. 
• Draw these 4 carbon atoms with out any bonds between them. 
• Number the carbon atoms from left to right
• This is shown in fig.14.51(a) below:

Fig.14.51

2. The suffix is yne. So there will be a triple bond.
• The position of the bond is given as 1 
• This indicates that, the bond is in between carbon atoms 1 and 2
    ♦ There is no other possibility because, 1 is the lowest possible number
• So draw a triple bond between carbon atoms 1 and 2
    ♦ All other carbon-carbon bonds can be shown as single bonds
• This is shown in fig.14.51(b)
3. Fill all the valencies of carbon atoms. 
• Since it is a hydrocarbon, only carbon and hydrogen will be present. 
    ♦ So we use hydrogen to fill up the valencies. 
• The result is shown in fig.14.51(c) above. So this fig.14.51(c) shows the required structure
• Note that, there is only one hydrogen atom for the first carbon and no hydrogen for the second carbon. Reader may write the reason for this in his/her own notebooks.

---

Now we will see some solved examples based on what we have discussed in this section and the previous section

Solved example 14.4:
Write the IUPAC name of the compound shown in fig.14.52(a) below:

Fig.14.52

Solution:
1. Two possible methods of 'numbering of carbon atoms' are shown in figs.(b) and (c)
• In fig.(b), the numbering is done from left to right
    ♦ In this, the 'carbon atoms linked to the double bond' gets 4 and 5
• In fig.(c), the numbering is done from right to left
    ♦ In this, the 'carbon atoms linked to the double bond' gets 2 and 3
2. According to the IUPAC rules, 'carbon atoms linked to the double bond' should get the lowest number. In our present problem, 
• The numbering in fig.(b) gives 4 and 5
• The numbering in fig.(c) gives 2 and 3
3. The numbering in fig.(c) gives the lowest number 2. So it is the correct method of numbering.
4. Now we can assemble the name. The rule for assembling is:
Word root+hyphen+position of double bond+hyphen+suffix
• As there are 6 carbon atoms, the word root is hex
• As one carbon-carbon bond is a double bond, the suffix is ene 
So we get: hex-2-ene.

Solved example 14.5
Given: IUPAC name is hex-3-yne. Draw the structure
Solution:
We will write the procedure in steps:
1. The word root is hex. So there will be 6 carbon atoms. 
• Draw these 6 carbon atoms with out any bonds between them. 
• Number the carbon atoms from left to right
• This is shown in fig.14.53(a) below:

Fig.14.53

2. The suffix is yne. So there will be a triple bond.
• The position of the bond is given as 3 
• This indicates that, the bond is in between carbon atoms 3 and 4
    ♦ The bond cannot be in between carbon atoms 2 and 3
    ♦ This is because, the IUPAC name would then be hex-2-yne
• So draw a triple bond between carbon atoms 3 and 4
    ♦ All other carbon-carbon bonds can be shown as single bonds
• This is shown in fig.14.53(b)
3. Fill all the valencies of carbon atoms. 
• Since it is a hydrocarbon, only carbon and hydrogen will be present. 
    ♦ So we use hydrogen to fill up the valencies. 
• The result is shown in fig.14.53(c) above. So this fig.14.53(c) shows the required structure
• Note that, there is no hydrogen for the third and fourth carbons. Reader may write the reason for this in his/her own notebooks.

---

In the next section, we will see functional groups.

PREVIOUS      CONTENTS       NEXT

                        Copyright©2017 High school Chemistry lessons. blogspot.in - All Rights Reserved

Chapter 8 - Carbon and it's compounds

In the previous section, we completed the discussion on the compounds of some non-metals. In this section we will discuss about Carbon and it's compounds.

Importance of Carbon

• We know that all substances around us are made up of chemical compounds. 
• These chemical compounds are made up of elements. 
■ If we make a list of ten things that we use in our day to day life, most of the items in that list will contain carbon.

Food items, clothes, oils, soaps, cosmetics, fuels, paints, rubber, paper etc., are all made up of compounds of carbon. In addition to this, plant and animal bodies are made up of carbon compounds. New compounds of carbon are being discovered or created artificially almost every day.

Bonding between atoms in carbon compounds

We know that, any compound will have atoms of various elements. A carbon compound will have atoms of carbon and one or more other elements. The other elements may be hydrogen, oxygen, chlorine etc., Now we will discuss how the atoms are held together in a molecule of a carbon compound.

The atomic number of carbon is 6. It's electronic configuration is 2,4. So it will have 4 valence electrons. 
To attain the stable electronic configuration: 
• the carbon should lose the 4 electrons. 
• or, it should gain 4 new electrons. 
Both the above options are difficult. Let us see the reason:
■ A large amount of energy will be required to remove the four valence electrons. Even if they are removed, the 6 protons in the nucleus will be attracting just 2 electrons. This is not possible. So, removal of 4 electrons will not occur
■ If the carbon atom is to gain 4 electrons, the 6 protons in the nucleus will have to attract and hold 10 electrons in the shells. This is difficult. So gaining of electrons also will not occur

What occurs is the sharing of electrons. This will give rise to covalent bonds between carbon atom and atoms of other elements. We have seen the details about covalent bonds earlier. Let us now draw the covalent bonds in some of the carbon compounds:
■ First we will take Methane (CH₄)
We have already seen it when we discussed about covalent bond. The details are presented here again.
• Carbon has an atomic number 6. It’s electronic configuration is 2,4. It has 4 electrons in the outer most shell. It needs four more electrons to attain octet. 
• Hydrogen has an atomic number 1. It’s electronic configuration is also 1. It has 1 electron in the outer most shell. It needs one more electron to attain octet.
• We see that both carbon and hydrogen are in ‘need for electrons’. They cannot donate any electrons. So the only solution is to form a covalent bond as shown in fig.8.1 below:

Fig.8.1

• The details about this bond can be written as:
• One carbon atom combines with 4 hydrogen atoms. Each of the four hydrogen atoms share one pair of electrons with the carbon atom
• In total, four pairs (that is., eight electrons) are shared. The eight electrons in the pairs, belongs to both the carbon atom and the hydrogen atom.
• The carbon atom now has the required four electrons for octet. 
• The hydrogen atom now has the required 2 electrons for octet
• As the pair belongs to both the atoms, they will not be able to move away from each other
• That is., the carbon and four hydrogen atoms will have to always stick together. Thus a bond is formed between the five atoms
• In each bond, there is a sharing of one pair of electrons. So each bond is a 'single bond'. So methane can be represented as shown in fig.8.2(a) below. The 'single lines' between C and H indicates 'single bonds'.

Fig.8.2

■ Next we will consider Carbon dioxide (CO₂)
• Carbon has an atomic number 6. It’s electronic configuration is 2,4. It has 4 electrons in the outer most shell. It needs four more electrons to attain octet. 
• Oxygen has an atomic number 8. It’s electronic configuration is 2,6. It has 6 electrons in the outer most shell. It needs two more electrons to attain octet.
• We see that both carbon and oxygen are in ‘need for electrons’. They cannot donate any electrons. So the only solution is to form a covalent bond as shown in fig.8.3 below:

Fig.8.3

• The details about this bond can be written as:
• One carbon atom combines with 2 oxygen atoms. Each of the two oxygen atoms share two pairs of electrons with the carbon atom
• In total, four pairs (that is., eight electrons) are shared. The eight electrons in the pairs, belongs to both the carbon atom and the oxygen atoms.
• The carbon atom now has the required 4 electrons for octet. 
• The oxygen atoms now has the required 2 electrons for octet
• As the pair belongs to both the atoms, they will not be able to move away from each other
• That is., the carbon and two oxygen atoms will have to always stick together. Thus a bond is formed between the three atoms
• In each bond, there is a sharing of two pairs of electrons. Thus each bond is a 'double bond'. So carbon dioxide can be represented as shown in fig.8.2(b) above. The 'double lines' between C and O indicates 'double bonds'.

A very large number of carbon compounds

We have seen that many objects that use in our day to day life are made up of carbon compounds. The number of carbon compounds was recently estimated to be nearly three million. That is., thirty lakhs. This is far greater than the total number of compounds formed by all other elements.

How is it possible for the carbon to form such large number of compounds? 
Let us analyse the reasons:
Reason 1
We know that carbon has 4 electrons in it's outer most shell. That is., it has a valency of 4. This makes it possible to form 3 different types of bonds. They are shown in fig.8.4 below:

Fig.8.4

• In the fig., the green dots indicate the electrons already possessed by the carbon atom. 
• The red dots indicate the new electrons which are acquired by sharing.
Let us see each type in detail:
1. In fig.8.4(a), the carbon is in bond with 4 other atoms. Each bond is a single bond. Through this arrangement, 4 new electrons are acquired. Thus octet is achieved.
2. In fig.b, the carbon is in bond with 3 other atoms. The bond with one of them is a double bond. Through this arrangement, 4 new electrons are acquired. Thus octet is achieved.
3. In fig.c, the carbon is in bond with 2 other atoms. The bond with one of them is a triple bond. Through this arrangement, 4 new electrons are acquired. Thus octet is achieved.
■ Thus there are three methods by which a carbon atom can enter into bond with others. The bonds so formed are very strong. The reason for the strong bonds can be explained as follows:
• The size of a carbon atom is small. So the force of attraction between the central nucleus and the outer most electrons will be strong. This gives strong bonds. So the molecules thus formed are very stable. 
The chemical properties of the newly formed molecules will depend upon the properties of carbon and also the newly bonded elements.
Reason 2
Carbon has the unique ability to enter into bondage with other carbon atoms. 
• Because of this ability, long chains of carbon atoms can be formed. This is shown in fig.8.5(a) below:

Fig.8.5

 • From a chain, another chain can branch off as shown in fig.b.
• The end carbon in a chain may enter into bondage with the first carbon to form a ring. This is shown in fig.c.
■The ability of atoms of an element to combine among themselves and form chains is called catenation.  Chains, branched chains and rings are formed because of this property. 
• Carbon has the ability to catenate. 
• But it is not seen in other elements. Silicon some times forms chains up to a maximum of  8 atoms long. But the molecules thus formed are very reactive. 
• We have seen that the carbon bond is very strong and hence stable. So a large variety of carbon compounds are possible. 


So we have seen two reasons. The formation of very large number of carbon compounds can be explained using those two reasons.

---

In the next section, we will see the actual arrangement of atoms in a molecule of Carbon compounds. 

PREVIOUS      CONTENTS       NEXT

                        Copyright©2017 High school Chemistry lessons. blogspot.in - All Rights Reserved

Chapter 3.2 - Covalent bond

In the previous section, we completed the discussion on ionic bond.  In this section, we will learn about covalent bond.

Covalent bonding

We have seen that the atoms attain octet by donating or accepting electrons. When the donation and acceptance takes place, cations and anions are formed. These ions will then be held together by the electrostatic force of attraction. And thus a molecule is formed. We also learned that, the above type of bonding between atoms is called Ionic bond.

But this type of bonding is not always possible. Consider the case of fluorine. It has a configuration 2,7. It has 7 electrons in the outer most shell. So it needs one more electron to attain octet. What if the atom available nearby is also fluorine? Both the atoms will be 'in need of an electron'. Neither one of them will be able to donate an electron. In such cases, sharing of electrons takes place. One pair of electrons is shared between two fluorine atoms. This is shown in the fig.3.11 below:

Fig.3.11

• Note that, one pair (that is., two electrons) is shared. The two electrons in the pair, belongs to both the atoms. So altogether, each atom will have 8 electrons in the outer most shell. 
• As the pair belongs to both the atoms, the two atoms will not be able to move away from each other
• That is., the two fluorine atoms will have to always stick together. Thus a bond is formed between the two fluorine atoms

---

The chemical bond formed as a result of the sharing of electrons between combining atoms is called covalent bond.

---

Since one pair of electrons is shared between two fluorine atoms, it is called a single bond. A single bond is represented by a small line between the symbols of the combining atoms. So the single bond between fluorine atoms can be represented as: F-F 

Another example: Consider chlorine with atomic number 17. It has a configuration 2,8,7. It has 7 electrons in the outer most shell. So it is in need of an electron to attain octet. It is a case similar to fluorine: ‘need of a single electron’. If we have two chlorine atoms, neither one of them will be able to donate an electron. So the only way to attain octet is to form a covalent bond as shown in fig.3.12 below:

Fig.3.12

The details about this bond can be written as:
• One pair (that is., two electrons) is shared. The two electrons in the pair, belongs to both the chlorine atoms. So altogether, each atom will have 8 electrons in the outer most shell. 
• As the pair belongs to both the atoms, the two atoms will not be able to move away from each other
• That is., the two chlorine atoms will have to always stick together. Thus a bond is formed between the two chlorine atoms
• Since one pair of electrons is shared between two chlorine atoms, it is a single bond. It is represented as: Cl-Cl

Another example: Consider oxygen with atomic number 8. It has a configuration 2,6. It has 6 electrons in the outer most shell. So it is in need of two electrons to attain octet. If we have two oxygen atoms, neither one of them will be able to donate two electrons. So the only way to attain octet is to form a covalent bond as shown in fig.3.13 below:

Fig.3.13

The details about this bond can be written as:
• Two pairs (that is., four electrons) are shared. The four electrons in the pairs, belongs to both the oxygen atoms. So altogether, each atom will have 8 electrons in the outer most shell. 
• As the pairs belongs to both the atoms, the two atoms will not be able to move away from each other
• That is., the two oxygen atoms will have to always stick together. Thus a bond is formed between the two oxygen atoms
• Since two pairs of electrons are shared between two oxygen atoms, it is called a double bond. A double bond is represented by two small lines between the symbols of the combining atoms. So the double bond between oxygen atoms can be represented as: O=O

Another example: Consider nitrogen with atomic number 7. It has a configuration 2,5. It has 5 electrons in the outer most shell. So it is in need of three electrons to attain octet. If we have two nitrogen atoms, neither one of them will be able to donate three electrons. So the only way to attain octet is to form a covalent bond as shown in fig.3.14 below:

Fig.3.14

The details about this bond can be written as:
• Three pairs (that is., six electrons) are shared. The six electrons in the pairs, belongs to both the nitrogen atoms. So altogether, each atom will have 8 electrons in the outer most shell. 
• As the pairs belongs to both the atoms, the two atoms will not be able to move away from each other
• That is., the two nitrogen atoms will have to always stick together. Thus a bond is formed between the two nitrogen atoms
• Since three pairs of electrons are shared between two nitrogen atoms, it is called a triple bond. A triple bond is represented by three small lines between the symbols of the combining atoms. So the triple bond between nitrogen atoms can be represented as: N≡N

---

So we have seen that a covalent bond may be a single, double or triple bond. It is related to the ‘number of pairs’ of electrons shared. This will be more clear from the following table:

---

The covalent bonds that we have seen so far, are formed between atoms of the same element. In the above cases, two atoms of the same element combined to form a molecule. Thus, two individual atoms, who would be unstable by themselves, attained stability by forming a covalent bond between them. It may be noted that, since two atoms are present, it is a diatomic molecule. We have see the basics about mono, di and poly atomic molecules here.

---

We will now see a molecule formed from two different atoms. Consider the formation of  hydrogen chloride.

• Hydrogen has an atomic number 1. It’s electronic configuration is also 1. It has 1 electron in the outer most shell. It needs one more electron to attain octet. 
• Chlorine has an atomic number 17. It’s electronic configuration is 2,8,7. It has 7 electrons in the outer most shell. It also needs one more electron to attain octet.
• We see that both hydrogen and chlorine are in need for a single electron. They cannot donate any electrons. So the only solution is to form a covalent bond as shown in fig.3.15 below:

Fig.3.15

The details about this bond can be written as:
• One pair (that is., two electrons) is shared. The two electrons in the pair, belongs to both the hydrogen atom and the chlorine atom.
• The hydrogen atom now has the required two electrons for octet. 
• The chlorine atom now has the required 8 electrons for octet
• As the pair belongs to both the atoms, the two atoms will not be able to move away from each other
• That is., the hydrogen and chlorine atoms will have to always stick together. Thus a bond is formed between the two atoms
• Since one pair of electrons is shared, it is a single bond. It is represented as: H-Cl

Another example: Consider the formation of carbontetrachloride
• Carbon has an atomic number 6. It’s electronic configuration is 2,4. It has 4 electrons in the outer most shell. It needs four more electrons to attain octet. 
• Chlorine has an atomic number 17. It’s electronic configuration is 2,8,7. It has 7 electrons in the outer most shell. It needs one more electron to attain octet.
• We see that both carbon and chlorine are in ‘need for electrons’. They cannot donate any electrons. So the only solution is to form a covalent bond as shown in fig.3.16 below:

Fig.3.16

• The details about this bond can be written as:
• One carbon atom combines with 4 chlorine atoms. Each of the four chlorine atoms share one pair of electrons with a carbon atom
• In total, four pairs (that is., eight electrons) are shared. The eight electrons in the pairs, belongs to both the carbon atom and the chlorine atom.
• The carbon atom now has the required 8 electrons for octet. 
• The chlorine atom now has the required 8 electrons for octet
• As the pair belongs to both the atoms, the two atoms will not be able to move away from each other
• That is., the carbon and chlorine atoms will have to always stick together. Thus a bond is formed between the five atoms 

---

We will now see some solved examples

Solved example 3.4

Illustrate the chemical bond in the following covalent compounds using electron dot diagram.

(i) CH₄,  (ii) HF,  (iii) H₂O

Solution:

• (i) CH₄: Carbon has an atomic number 6. It’s electronic configuration is 2,4. It has 4 electrons in the outer most shell. It needs four more electrons to attain octet. 
• Hydrogen has an atomic number 1. It’s electronic configuration is also 1. It has 1 electron in the outer most shell. It needs one more electron to attain octet.
• We see that both carbon and hydrogen are in ‘need for electrons’. They cannot donate any electrons. So the only solution is to form a covalent bond as shown in fig.3.17 below:

Fig.3.17

• The details about this bond can be written as:
• One carbon atom combines with 4 hydrogen atoms. Each of the four hydrogen atoms share one pair of electrons with a carbon atom
• In total, four pairs (that is., eight electrons) are shared. The eight electrons in the pairs, belongs to both the carbon atom and the hydrogen atom.
• The carbon atom now has the required 4 electrons for octet. 
• The hydrogen atom now has the required 2 electrons for octet
• As the pair belongs to both the atoms, the two atoms will not be able to move away from each other
• That is., the carbon and hydrogen atoms will have to always stick together. Thus a bond is formed between the five atoms

• (ii) HF: Hydrogen has an atomic number 1. It’s electronic configuration is also 1. It has 1 electron in the outer most shell. It needs one more electron to attain octet. 
• Fluorine has an atomic number 9. It’s electronic configuration is 2,7. It has 7 electrons in the outer most shell. It also needs one more electron to attain octet.
• We see that both hydrogen and fluorine are in need for a single electron. They cannot donate any electrons. So the only solution is to form a covalent bond as shown in  fig.3.18 below:

Fig.3.18

The details about this bond can be written as:
• One pair (that is., two electrons) is shared. The two electrons in the pair, belongs to both the hydrogen atom and the fluorine atom.
• The hydrogen atom now has the required two electrons for octet. 
• The fluorine atom now has the required 8 electrons for octet
• As the pair belongs to both the atoms, the two atoms will not be able to move away from each other
• That is., the hydrogen and fluorine atoms will have to always stick together. Thus a bond is formed between the two atoms
• Since one pair of electrons is shared, it is a single bond. It is represented as: H-F

• (iii) H₂O: Oxygen has an atomic number 8. It’s electronic configuration is 2,6. It has 6 electrons in the outer most shell. It needs two more electrons to attain octet. 
• Hydrogen has an atomic number 1. It’s electronic configuration is also 1. It has 1 electron in the outer most shell. It needs one more electron to attain octet.
• We see that both oxygen and hydrogen are in ‘need for electrons’. They cannot donate any electrons. So the only solution is to form a covalent bond as shown in  fig.3.19 below:

Fig.3.19

• The details about this bond can be written as:
• One oxygen atom combines with 2 hydrogen atoms. Each of the 2 hydrogen atoms share one pair of electrons with a oxygen atom
• In total, two pairs (that is., four electrons) are shared. The four electrons in the pairs, belongs to both the oxygen atom and the hydrogen atom.
• The oxygen atom now has the required 8 electrons for octet. 
• The hydrogen atom now has the required 2 electrons for octet
• As the pair belongs to both the atoms, the two atoms will not be able to move away from each other
• That is., the oxygen and hydrogen atoms will have to always stick together. Thus a bond is formed between the three atoms.

---

In the next section, we will learn about electronegativity.

PREVIOUS      CONTENTS       NEXT

                        Copyright©2016 High school Chemistry lessons. blogspot.in - All Rights Reserved