Question

We saw in Section 8 - 15 that addition of \(\mathrm{HBr}\) to a terminal alkyne leads to the Markovnikov addition product, with the Br bonding to the more highly substituted carbon. How could you use \({ }^{13} \mathrm{C}\) NMR to identify the product of the addition of 1 equivalent of HBr to hex-1-yne?

Short answer

Use {¹³}C NMR to identify the significant downfield shift of C2, confirming formation of 2-bromohexene.

Step-by-step solution

Understand Reactants and Product Formation

The reaction involves the addition of HBr to hex-1-yne, an alkyne located at the end of a carbon chain. According to Markovnikov's rule, bromine (Br) will attach to the more substituted carbon atom. In hex-1-yne, after the addition of one equivalent of HBr, the triple bond between C1 and C2 will become a double bond, with Br attaching to C2, forming 2-bromohexene.

Predict the Carbon Environment Changes

In the starting alkyne (hex-1-yne), the carbon at the triple bond (C1 and C2) are sp hybridized. After the reaction, C2 becomes more substituted and sp2 hybridized due to the addition of Br, while C1 remains unsaturated but changes to sp2 hybridization due to the remaining double bond.

Analyze {¹³}C NMR Shifts

{¹³}C NMR can identify different carbon environments by shifts in resonance frequencies. sp2 hybridized carbons appear downfield (higher ppm) compared to sp hybridized ones. In the product, C2 now bonded with Br will show a significant downfield shift compared to other carbons, serving as an indicator of forming 2-bromohexene.

Identify the Characteristic NMR Signals

In the {¹³}C NMR spectrum, look for a distinct signal corresponding to C2, which has shifted due to Br addition – typically appearing between 30-70 ppm depending on the solvent and instrument used, marking its identity as part of the Markovnikov addition product.

Key concepts

Markovnikov Addition

Markovnikov's rule is a guiding principle for understanding the addition of hydrogen halides such as HBr to alkenes and alkynes. It states that in the addition of HX (where X is a halogen) to an unsymmetrical alkene or alkyne, the hydrogen (H) atom will attach to the carbon with more hydrogen atoms already attached, and the halogen will attach to the more substituted carbon. Markovnikov addition helps predict the major product in such reactions.
In the context of terminal alkynes, such as hex-1-yne, only one end of the triple-bond is terminal. Here, when HBr is added, bromine attaches to the more substituted carbon in the chain. For example, in hex-1-yne, after the reaction, Br will bond with the carbon-2 (C2) instead of the less substituted carbon-1 (C1). It results in a more stable carbocation intermediate, leading to the predominant formation of 2-bromohexene.
This rule provides a useful prediction tool for chemists and helps in designing synthetic pathways for organic compounds.

Alkyne Reactions

Alkynes are hydrocarbons containing carbon-carbon triple bonds, and their reactivity is distinct from that of alkenes or alkanes. Reactions with alkynes can include hydrogenation, halogenation, and hydrohalogenation among others. They engage actively with molecules like HBr and follow principles like Markovnikov's rule.
In the specific case of hex-1-yne reacting with HBr, the molecule undergoes hydrohalogenation, where one equivalent of HBr adds across the triple bond. This results in the conversion of the triple bond into a double bond, forming an alkene. The product of this particular reaction is 2-bromohexene, representing a typical product of Markovnikov addition.
Such transformations are key reactions in organic synthesis, allowing the modification of carbon skeletons and the addition of functional groups like bromine.

Carbon Hybridization

Carbon hybridization is an essential concept to understand the electronic structure and geometry around carbon atoms in organic molecules. It explains how carbon atoms bond with other atoms in varied configurations.
Original sp hybridization in a terminal alkyne like hex-1-yne involves linear arrangement of atoms with 180-degree bond angles. The addition of HBr changes these carbon atoms' hybridization state from sp to sp2. The newly formed double-bonded product, 2-bromohexene, features sp2 hybridization for carbons 1 and 2. This allows them to adopt a planar geometry and influences the chemical properties and further reactivity of the molecule.
Understanding hybridization helps in predicting molecular shapes, bond angles, and other aspects of the molecular geometry, which are crucial for interpreting reaction mechanisms and designing chemical compounds.

NMR Chemical Shifts

Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful analytical technique used to determine the structure of organic compounds. In {¹³}C NMR spectroscopy, chemical shifts arise due to the different electronic environments surrounding carbon atoms.
Carbon atoms in different hybridization states show distinct signals in an NMR spectrum. Typically, sp-hybridized carbons are upfield (lower ppm), while sp2 carbons appear downfield (higher ppm). After the Markovnikov addition in the hex-1-yne reaction, significant chemical shifts help identify the product as 2-bromohexene.
For instance, the sp2 hybridized carbon (C2) bonded to the bromine atom will typically show a chemical shift in the range of 30-70 ppm. This shift is indicative of electronegative atoms like bromine influencing the electronic environment but varies depending on the solvent and specific NMR settings used.
Recognizing these shifts allows chemists to distinguish different carbon environments and confirm the structure of synthesized molecules.