Q2P - Organic Chemistry (Mcmurry)

Question

Use the d1/d2 convention to indicate the direction of expected polarity for each of the bonds indicated.

(a) H₃C–Cl (b) H₃C–NH₂ (c) H₂N–H

(d) H₃C–SH (e) H₃C–MgBr (f) H₃C–F

Step-by-Step Solution

Answer

The partial negative charge is placed on the more electronegative atom, and partial positive charge is placed on the less electronegative atom.

                                                                                                   

    (a)   $H_3{C}^{\delta +}-C{l}^{\delta -}$     (b)   $H_3{C}^{\delta +}-N{H_2}^{\delta -}$        (c)$H_2{N}^{\delta +}-H^{\delta -}$

 

 

                                                                                                       

       (d)    ${\mathrm{H}}_3\mathrm{C}-\mathrm{SH}$                          (e)  data-custom-editor="chemistry" ${\mathrm{H}}_3{\mathrm{C}}^{\delta -}-{\mathrm{MgBr}}^{\delta +}$             (f)    data-custom-editor="chemistry" ${\mathrm{H}}_3{\mathrm{C}}^{\delta +}-{\mathrm{F}}^{\delta +}$

 

Chemical Polarity of an atom

Polarity is a separation of electric charge leading to a molecule or its chemical group having an electric dipole moment, with a negatively charged and positively charged end. Polar molecules must contain one or more polar bonds due to difference in electronegativity between the bonded atoms. It also occurs when two atom do not share electron equally in covalent bond.

Electronegativity is a chemical property that describes the tendency of an atom or functional group to attract electron toward itself.it is dimensionless property because it is only a tendency.

Periodic trends in the electronegativity of an elements to determine polarity.

As we move across a period from left to right the nuclear charge increases and the atomic size decreases, therefore the value of electronegativity increase across a period in periodic table.

There is also increase in the atomic number as we move down the group in the modern periodic table. The nuclear charge also increases but effect of increase in nuclear charge is overcome by the addition of one shell. Hence the value of electronegativity decreases as we move down the group.

Indicating polarity for each bond using d1/d2 convention.

The d1/d2 i.e. (delta+/delta-) convention is a convention to indicate direction of expected polarity of each bond.

                                                                                                   

     (a)  $H_3{C}^{\delta +}-C{l}^{\delta -}$                        (b)  ${\mathrm{H}}_3{\mathrm{C}}^{\delta +}-{{\mathrm{NH}}_2}^{\delta -}$         (c) ${\mathrm{H}}_2{\mathrm{N}}^{\mathrm{\delta }+}-{\mathrm{H}}^{\mathrm{\delta }-}$

 

 

                                                                                                       

      (d)  ${\mathrm{H}}_3\mathrm{C}-\mathrm{SH}$                              (e)    ${\mathrm{H}}_3{\mathrm{C}}^{\delta -}-{\mathrm{MgBr}}^{\delta +}$      (f) ${\mathrm{H}}_3{\mathrm{C}}^{\delta +}-{\mathrm{F}}^{\delta -}$     

 

      Carbon and Sulphur 

         have identical

       electronegativities.
 

 

 

 

 

(a)${\mathrm{CH}}_3\mathrm{Cl}$ is polar molecule. The hydrogen at the molecule is less electronegative than carbon and so is slightly positive. This means the molecule now has slightly positive carbon and a slightly negative chlorine atom.

(b).$C{H}_{3}N{H}_2$is also a polar molecule. The hydrogen at the molecule is less electronegative than carbon and so is slightly positive. This means the molecule now has slightly positive carbon and a slightly negative Nitrogen atom.

 

(c).$\mathrm{\mathbb{N}}{\mathrm{H}}_3$In molecule electronegativity of nitrogen is more than hydrogen due to increase in atomic number electronegativity increase.

(d). It is a non-polar molecule because carbon and Sulphur have identical electronegativities.

 

(e) A Grignard reagent ${\mathrm{CH}}_3\mathrm{MgBr}$ has a very polar carbon and magnesium bond. It is also a general observation that metals show a lower value of electronegativity as compare to the non-metals.so, the carbon atom has a negative partial charge and the metal a positive charge.

(f) In ${\mathrm{CH}}_3\mathrm{F}$ molecule electronegativity of fluorine is more than carbon due to increase in atomic number electronegativity increases. The hydrogen at the molecule is less electronegative than carbon and so is slightly positive. This means the molecule now has slightly positive carbon and a slightly negative fluorine atom.