Problem 72 - Introductory Chemistry Atoms First

Question

Give the molecular formula and line drawing for: (a) propene (b) 1 -octene (c) hexane (d) 2 -butyne (e) 1 -butyne (f) 2,3 -dimethylbutane

Step-by-Step Solution

Answer

(a) C3H6, \[H_2C=CHCH_3\] (b) C8H16, \[CH_2=CH(CH_2)_6H\] (c) C6H14, \[CH_3(CH_2)_4CH_3\] (d) C4H6, \[CH_3C\equiv CCH_3\] (e) C4H6, \[HC\equiv CCH_2CH_3\] (f) C6H14, \[CH_3C(CH_3)_2CH_2CH_3\]

Step 1: Determine the number of carbon atoms and type of bond

'Prop' in propene refers to three carbon atoms, and 'ene' indicates the presence of a double bond between the carbon atoms.

Step 2: Write the molecular formula

The molecular formula for propene is C3H6.

Step 3: Draw the line structure

Line structure for propene: \[H_2C=CHCH_3\] (b) 1-Octene

Step 4: Determine the number of carbon atoms and type of bond

'Oct' in 1-octene refers to eight carbon atoms, and 'ene' indicates the presence of a double bond between carbon atoms. The number '1' indicates the location of the double bond.

Step 5: Write the molecular formula

The molecular formula for 1-octene is C8H16.

Step 6: Draw the line structure

Line structure for 1-octene: \[CH_2=CH(CH_2)_6H\] (c) Hexane

Step 7: Determine the number of carbon atoms and type of bond

'Hex' in hexane refers to six carbon atoms, and 'ane' indicates all single bonds between carbon atoms.

Step 8: Write the molecular formula

The molecular formula for hexane is C6H14.

Step 9: Draw the line structure

Line structure for hexane: \[CH_3(CH_2)_4CH_3\] (d) 2-Butyne

Step 10: Determine the number of carbon atoms and type of bond

'But' in 2-butyne refers to four carbon atoms, and 'yne' indicates the presence of a triple bond between carbon atoms. The number '2' indicates the location of the triple bond.

Step 11: Write the molecular formula

The molecular formula for 2-butyne is C4H6.

Step 12: Draw the line structure

Line structure for 2-butyne: \[CH_3C\equiv CCH_3\] (e) 1-Butyne

Step 13: Determine the number of carbon atoms and type of bond

'But' in 1-butyne refers to four carbon atoms, and 'yne' indicates the presence of a triple bond between carbon atoms. The number '1' indicates the location of the triple bond.

Step 14: Write the molecular formula

The molecular formula for 1-butyne is C4H6.

Step 15: Draw the line structure

Line structure for 1-butyne: \[HC\equiv CCH_2CH_3\] (f) 2,3-Dimethylbutane

Step 16: Determine the number of carbon atoms and type of bond

'But' in 2,3-dimethylbutane refers to four carbon atoms in the main chain, and 'ane' refers to all single bonds between carbon atoms. 'Dimethyl' indicates that there are two methyl groups attached to the main carbon chain at positions 2 and 3.

Step 17: Write the molecular formula

The molecular formula for 2,3-dimethylbutane is C6H14.

Step 18: Draw the line structure

Line structure for 2,3-dimethylbutane: \[CH_3C(CH_3)_2CH_2CH_3\]

Key Concepts

Alkenes

Alkenes are a class of hydrocarbons that contain at least one carbon-to-carbon double bond. Their general formula is C_nH_2n, where 'n' represents the number of carbon atoms. For instance, propene and 1-octene from our exercise are both alkenes.

Propene (C₃H₆) includes a double bond between two of the three carbon atoms, while 1-octene (C₈H₁₆) has a double bond right at the first carbon in the chain. The position of the double bond can change the properties of the molecule, making the awareness of structure critical to understanding alkenes.

Alkynes

Alkynes, on the other hand, are characterized by the presence of a carbon-to-carbon triple bond. They follow the general formula C_nH_2n-2. In the given exercises, 2-butyne (C₄H₆) and 1-butyne (C₄H₆) are examples of alkynes.

The position of the triple bond significantly affects the molecule's reactivity. In 2-butyne, the triple bond is between the second and third carbon atoms, while in 1-butyne, it is between the first and second carbon atoms. The naming of alkynes is similarly essential to alkenes because it affects how these molecules interact in chemical reactions.

Hydrocarbons

Hydrocarbons are compounds composed entirely of carbon (C) and hydrogen (H) atoms. They are classified into alkanes, alkenes, and alkynes, based on the type of bonding between carbon atoms. Alkanes have all single bonds, alkenes have at least one double bond, and alkynes have at least one triple bond.

In our exercise, hexane (C₆H₁₄) is an alkane with single bonds only, making it a saturated hydrocarbon. Saturated hydrocarbons tend to be less reactive than unsaturated hydrocarbons, like alkenes and alkynes, which have double and triple bonds, respectively.

Line Structure Representations

Line structure representations provide a simplified way to draw organic molecules. In these diagrams, carbon atoms are represented by the ends of lines or where lines intersect. Hydrogen atoms attached to carbons are usually not shown for simplicity unless they are part of a functional group.

The line structures of propene, 1-octene, hexane, 2-butyne, 1-butyne, and 2,3-dimethylbutane were given in the exercise. These structural formulas are vital for visualizing the arrangement of atoms within the molecule, which is important when predicting the physical and chemical properties of the molecule.

Chemical Bonding

Chemical bonding is the force that holds atoms together in molecules. In hydrocarbons, carbon atoms can form four covalent bonds with other atoms. Single, double, and triple bonds correspond to the number of shared electron pairs between two atoms.

Single bonds, as found in alkanes, allow for free rotation around the bond axis, whereas double and triple bonds found in alkenes and alkynes, respectively, restrict this rotation, leading to different molecular shapes and chemical reactivities. A deep understanding of chemical bonding is crucial for predicting how molecules will interact during chemical reactions.