{"id":231717,"date":"2025-06-11T10:39:55","date_gmt":"2025-06-11T10:39:55","guid":{"rendered":"https:\/\/learnexams.com\/blog\/?p=231717"},"modified":"2025-06-11T10:39:57","modified_gmt":"2025-06-11T10:39:57","slug":"spectrum-a-f-are-the-ir-spectra-of-the-compounds-shown","status":"publish","type":"post","link":"https:\/\/www.learnexams.com\/blog\/2025\/06\/11\/spectrum-a-f-are-the-ir-spectra-of-the-compounds-shown\/","title":{"rendered":"Spectrum A — F are the IR spectra of the compounds shown"},"content":{"rendered":"\n

Spectrum A — F are the IR spectra of the compounds shown. In Spectrum A — F, assign the major absorptions above ~1500 cm
in the spectrum of each compound, (using Tables 8.1 and 8.2 [see background information]). A. IR spectra of phenylethyne 100 Transmittance (%) 80 60 40 20 0 4000 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 Wavenumber (cm
) B. IR spectra of n-butyl acetate 100 Transmittance (%) 80 60 40 20 0 4000 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 Wavenumber (cm
)<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

The Correct Answer and Explanation is:<\/mark><\/strong><\/p>\n\n\n\n

Let’s analyze and assign the major IR absorptions above ~1500 cm\u207b\u00b9<\/strong> for the two given spectra:<\/p>\n\n\n\n

\n\n\n\n

A. IR Spectrum of Phenylethyne<\/strong><\/h3>\n\n\n\n

Structure<\/strong>: Phenylethyne (C\u2086H\u2085\u2013C\u2261CH) is an aromatic alkyne.<\/p>\n\n\n\n

Major IR Absorptions:<\/strong><\/p>\n\n\n\n

    \n
  1. ~3300 cm\u207b\u00b9<\/strong> \u2013 Sharp, strong \u2261C\u2013H stretch<\/strong> (sp-hybridized C\u2013H bond typical of terminal alkynes).<\/li>\n\n\n\n
  2. ~2100\u20132200 cm\u207b\u00b9<\/strong> \u2013 Weak C\u2261C stretch<\/strong> (typical alkyne triple bond stretching).<\/li>\n\n\n\n
  3. ~3030 cm\u207b\u00b9<\/strong> \u2013 Aromatic C\u2013H stretch<\/strong> (sp\u00b2 hybridized C\u2013H).<\/li>\n\n\n\n
  4. ~1600 and ~1500 cm\u207b\u00b9<\/strong> \u2013 Aromatic C=C stretching<\/strong> (conjugated ring modes).<\/li>\n<\/ol>\n\n\n\n

    These peaks confirm the presence of both an aromatic ring and a terminal alkyne.<\/p>\n\n\n\n

    \n\n\n\n

    B. IR Spectrum of n<\/em>-Butyl Acetate<\/strong><\/h3>\n\n\n\n

    Structure<\/strong>: CH\u2083COO(CH\u2082)\u2083CH\u2083 \u2013 an ester with an aliphatic chain.<\/p>\n\n\n\n

    Major IR Absorptions:<\/strong><\/p>\n\n\n\n

      \n
    1. ~1740 cm\u207b\u00b9<\/strong> \u2013 Strong, sharp C=O (carbonyl) stretch<\/strong> (typical for esters).<\/li>\n\n\n\n
    2. ~2950\u20132850 cm\u207b\u00b9<\/strong> \u2013 C\u2013H stretches<\/strong> (sp\u00b3 C\u2013H from aliphatic chains).<\/li>\n\n\n\n
    3. ~1240\u20131050 cm\u207b\u00b9<\/strong> \u2013 Strong, complex C\u2013O stretch<\/strong> (asymmetric\/symmetric ester vibrations).<\/li>\n\n\n\n
    4. ~1450 and ~1370 cm\u207b\u00b9<\/strong> \u2013 C\u2013H bending<\/strong> from \u2013CH\u2082 and \u2013CH\u2083 groups.<\/li>\n<\/ol>\n\n\n\n
      \n\n\n\n

      Explanation (300 words)<\/strong><\/h3>\n\n\n\n

      Infrared (IR) spectroscopy is a powerful technique used to identify functional groups within a molecule based on the absorption of infrared radiation, which causes molecular vibrations. Each functional group absorbs IR radiation at a characteristic wavenumber, typically measured in cm\u207b\u00b9.<\/p>\n\n\n\n

      In Spectrum A, the IR spectrum of phenylethyne<\/strong>, several key absorptions are noted. The sharp absorption near 3300 cm\u207b\u00b9<\/strong> indicates a terminal alkyne C\u2013H stretch (sp hybridized). The weak absorption near 2100\u20132200 cm\u207b\u00b9<\/strong> is consistent with a carbon\u2013carbon triple bond (C\u2261C) stretch. Additionally, aromatic C\u2013H stretching appears near 3030 cm\u207b\u00b9, and characteristic C=C stretches<\/strong> of the benzene ring occur near 1600 cm\u207b\u00b9 and 1500 cm\u207b\u00b9<\/strong>, confirming the presence of an aromatic ring.<\/p>\n\n\n\n

      In Spectrum B, the IR spectrum of n-butyl acetate<\/strong>, the strongest absorption appears at ~1740 cm\u207b\u00b9<\/strong>, which corresponds to the C=O stretching<\/strong> vibration of the ester functional group. The peaks between 2950\u20132850 cm\u207b\u00b9<\/strong> are due to aliphatic C\u2013H stretching<\/strong> from the butyl chain. The ester also exhibits C\u2013O stretching vibrations<\/strong> in the 1240\u20131050 cm\u207b\u00b9<\/strong> range. Additionally, C\u2013H bending modes<\/strong> near 1450 and 1370 cm\u207b\u00b9<\/strong> further support the presence of methyl and methylene groups.<\/p>\n\n\n\n

      By comparing IR absorption peaks to known reference values (Tables 8.1 and 8.2), the functional groups in both compounds are clearly identified. This makes IR spectroscopy an essential tool for structural determination in organic chemistry.<\/p>\n\n\n\n

      \"\"<\/figure>\n","protected":false},"excerpt":{"rendered":"

      Spectrum A — F are the IR spectra of the compounds shown. In Spectrum A — F, assign the major absorptions above ~1500 cmin the spectrum of each compound, (using Tables 8.1 and 8.2 [see background information]). A. IR spectra of phenylethyne 100 Transmittance (%) 80 60 40 20 0 4000 3600 3200 2800 2400 […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"site-sidebar-layout":"default","site-content-layout":"","ast-site-content-layout":"default","site-content-style":"default","site-sidebar-style":"default","ast-global-header-display":"","ast-banner-title-visibility":"","ast-main-header-display":"","ast-hfb-above-header-display":"","ast-hfb-below-header-display":"","ast-hfb-mobile-header-display":"","site-post-title":"","ast-breadcrumbs-content":"","ast-featured-img":"","footer-sml-layout":"","ast-disable-related-posts":"","theme-transparent-header-meta":"","adv-header-id-meta":"","stick-header-meta":"","header-above-stick-meta":"","header-main-stick-meta":"","header-below-stick-meta":"","astra-migrate-meta-layouts":"default","ast-page-background-enabled":"default","ast-page-background-meta":{"desktop":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}},"ast-content-background-meta":{"desktop":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}},"footnotes":""},"categories":[25],"tags":[],"class_list":["post-231717","post","type-post","status-publish","format-standard","hentry","category-exams-certification"],"_links":{"self":[{"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/posts\/231717","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/comments?post=231717"}],"version-history":[{"count":0,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/posts\/231717\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/media?parent=231717"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/categories?post=231717"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/tags?post=231717"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}