Question

What is wrong with each of the following structures? (a) \(\mathrm{CH}_{3}=\mathrm{HCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{C}=\mathrm{CH}_{2} \mathrm{CH}_{3}\)

Short answer

(a) Structure (a) violates element bonding rules; correct could be an alcohol or properly written alkene. (c) Needs removal for a carbon in the chain to adjust double bond, possibly 1-pentene.

Step-by-step solution

Analyze Structure (a)

The structure given is \( \text{CH}_3=\text{HCH}_2 \text{CH}_2 \text{OH} \). First, let's break it down to see if it follows the correct bonding rules for each element, particularly carbon and hydrogen. The structure implies a doubly-bonded carbon (alkene) group: \( \text{CH}_3=\text{HCH}_2\), which seems incorrect since no carbon is directly bonded to hydrogen negatively. This structure suggests a carbon bonded to hydrogen in a manner that is not possible because carbon forms a total of 4 bonds, and cannot form \( -\text{HCH}_2 \) in this linear fashion.

Correct the Structure (a)

The issue in \( \text{CH}_3=\text{HCH}_2 \text{CH}_2 \text{OH} \) is with the bonding pattern. The chemical consistent with the elements given would be \( \text{CH}_3\text{CH}_2\text{CH}_2\text{OH} \), an alcohol with all carbons single-bonded, which is butanol. Alternatively, if it were intended as an alkene, it might be \( \text{CH}_2=\text{CHCH}_2\text{OH} \), meaning an improper structure was drawn with a possible omittance.

Analyze Structure (c)

Consider the structure \( \text{CH}_3 \text{CH}_2 \text{C}=\text{CH}_2 \text{CH}_3 \). The given structure appears to have one too many carbon atoms, as the bond arrangement \( \text{C}=\text{C} \) already takes up 3 bonds; with \( \text{CH}_3 \) at each end, one available carbon bond site is filled.

Correct the Structure (c)

The structure \( \text{CH}_3 \text{CH}_2 \text{C}=\text{CH}_2 \text{CH}_3 \) can be corrected to a form consistent within the defined formulation; if we redefine it to \( \text{CH}_3 \text{CH}_2 \text{C}=\text{CH}_2 \), the bonding remains valid within known hydrocarbon chemistries, likely describing 1-pentene by earlier hydrogen misplacements or draws.

Key concepts

Chemical Bonding

Chemical bonding is the force that holds atoms together in a molecule. In organic chemistry, chemical bonds are typically covalent where electrons are shared between atoms. Understanding how these bonds form and what they can tell us about molecular structure is essential.
Carbon, which is a central element in organic compounds, typically forms four bonds due to its four valence electrons. This can be visualized by the use of Lewis structures which help illustrate bonding rules:

• Covalent bonds – where atoms share electrons.
• Double and triple bonds – more than one pair of shared electrons, common with carbon-carbon interactions as seen in alkenes.
• Non-bonding electron pairs (lone pairs) – electrons not involved in bonding.

Chemical bonding plays a crucial role in determining the structure and properties of organic compounds. By mastering these concepts, one can predict molecular shapes, and bond angles, and understand potential reactivity tendencies.

Alkenes

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond (C=C). This double bond introduces a degree of rigidity and plays a crucial role in defining the molecule's chemistry.
Here are some key features of alkenes:

• Unsaturated hydrocarbons – unlike alkanes, alkenes do not have the maximum number of hydrogen atoms possible.
• General formula – CnH2n, indicating less hydrogen compared to alkanes (CnH2n+2).
• Reactivity – the double bond is a site of high electron density making it reactive, especially to addition reactions.

An understanding of alkenes is crucial when drawing and verifying organic structures. Misplacement of double bonds can change the entire nature of the compound, as seen in the exercise solutions where recognizing incorrect double bond placements leads to error identification.

Molecular Structure

Molecular structure refers to the three-dimensional arrangement of atoms in a molecule. Understanding molecular structures allows chemists to predict physical and chemical properties. Here are some factors that influence molecular structure:

• Bond configurations – how the atoms are bonded whether single, double, or triple.
• Functional groups – certain groups of atoms that impart specific properties and reactions, like chains or hydroxyl groups in alcohols.
• Stereochemistry – especially important in compounds with the same molecular formula but different spatial arrangements.

When correcting molecular structures, like those in the exercises given, it’s essential to ensure that all valences are satisfied, functional groups are correctly placed, and the intended molecular geometry isn't altered.

Bonding Rules

Bonding rules govern how atoms combine to form molecules. In organic chemistry, these rules can often be visualized using simple models:

• The octet rule – atoms seek to complete their outer shell with eight electrons when forming organic molecules.
• Hydrogen forms one bond; it achieves a stable helium configuration with two electrons.
• Carbon, with four valence electrons, naturally forms four covalent bonds to satisfy the octet rule.

Correct application of these rules is vital. As observed in the exercise solutions, incorrect application leads to impossible structures with either too many or too few bonds. Following systematic approaches allows chemists to draw and interpret molecular structures accurately, avoiding common pitfalls.