Li, Cen et al. published their research in Huaxi Yaoxue Zazhi in 2009 | CAS: 58168-20-0

Ethyl 5-amino-4-cyano-3-(2-ethoxy-2-oxoethyl)thiophene-2-carboxylate (cas: 58168-20-0) belongs to nitriles. Nitrile compounds can be prepared by the incorporation of a cyanide source through C–C bond formation or by dehydration of primary carboxamides. Nitrile groups in organic compounds can undergo a variety of reactions depending on the reactants or conditions. A nitrile group can be hydrolyzed, reduced, or ejected from a molecule as a cyanide ion.Application of 58168-20-0

Synthesis of intermediate of strontium ranelate by using ionic liquid as phase transfer catalyst was written by Li, Cen;Jiang, Yi;Jiang, Ye;Liu, Feng-hua. And the article was included in Huaxi Yaoxue Zazhi in 2009.Application of 58168-20-0 This article mentions the following:

OBJECTIVE: To study a new catalytic way using ionic liquid as phase transfer catalyst for the synthesis of intermediate of strontium ranelate. METHODS: Tetra-Et ester of 5-aminothiophene was synthesized from 5-aminothiophene using Et bromoacetate as alkylating reagent and ionic liquid as phase transfer catalyst. RESULTS and CONCLUSION: The proposed method was not only suitable for industrial manufacture with high yield of 81%, but also could cut down the discharging of pollutants. In the experiment, the researchers used many compounds, for example, Ethyl 5-amino-4-cyano-3-(2-ethoxy-2-oxoethyl)thiophene-2-carboxylate (cas: 58168-20-0Application of 58168-20-0).

Ethyl 5-amino-4-cyano-3-(2-ethoxy-2-oxoethyl)thiophene-2-carboxylate (cas: 58168-20-0) belongs to nitriles. Nitrile compounds can be prepared by the incorporation of a cyanide source through C–C bond formation or by dehydration of primary carboxamides. Nitrile groups in organic compounds can undergo a variety of reactions depending on the reactants or conditions. A nitrile group can be hydrolyzed, reduced, or ejected from a molecule as a cyanide ion.Application of 58168-20-0

Referemce:
Nitrile – Wikipedia,
Nitriles – Chemistry LibreTexts

Jelier, Benson J. et al. published their research in Angewandte Chemie, International Edition in 2018 | CAS: 63968-85-4

2-(Trifluoromethoxy)benzonitrile (cas: 63968-85-4) belongs to nitriles. Nitrile function is a very important functional group because it can be manipulated to other functional groups such as carboxylic acid by hydrolysis or amine by reduction, respectively. Some nitriles are manufactured by heating carboxylic acids with ammonia in the presence of catalysts. This process is used to make nitriles from natural fats and oils, the products being used as softening agents in synthetic rubbers, plastics, and textiles and for making amines.Application of 63968-85-4

Radical Trifluoromethoxylation of Arenes Triggered by a Visible-Light-Mediated N-O Bond Redox Fragmentation was written by Jelier, Benson J.;Tripet, Pascal F.;Pietrasiak, Ewa;Franzoni, Ivan;Jeschke, Gunnar;Togni, Antonio. And the article was included in Angewandte Chemie, International Edition in 2018.Application of 63968-85-4 This article mentions the following:

A simple trifluoromethoxylation method enables non-directed functionalization of C-H bonds on a range of substrates, providing access to aryl trifluoromethyl ethers. This light-driven process is distinctly different from conventional procedures and occurs through an OCF3 radical mechanism mediated by a photoredox catalyst, which triggers an N-O bond fragmentation. The pyridinium-based trifluoromethoxylation reagent is bench-stable and provides access to synthetic diversity in lead compounds in an operationally simple manner. In the experiment, the researchers used many compounds, for example, 2-(Trifluoromethoxy)benzonitrile (cas: 63968-85-4Application of 63968-85-4).

2-(Trifluoromethoxy)benzonitrile (cas: 63968-85-4) belongs to nitriles. Nitrile function is a very important functional group because it can be manipulated to other functional groups such as carboxylic acid by hydrolysis or amine by reduction, respectively. Some nitriles are manufactured by heating carboxylic acids with ammonia in the presence of catalysts. This process is used to make nitriles from natural fats and oils, the products being used as softening agents in synthetic rubbers, plastics, and textiles and for making amines.Application of 63968-85-4

Referemce:
Nitrile – Wikipedia,
Nitriles – Chemistry LibreTexts

Chernyuk, O. M. et al. published their research in Ukrainskii Khimicheskii Zhurnal (Russian Edition) in 2009 | CAS: 70291-62-2

2-Amino-5,6-dihydro-4H-cyclopenta[b]thiophene-3-carbonitrile (cas: 70291-62-2) belongs to nitriles. The R-C-N bond angle in and nitrile is 180° which give a nitrile functional group a linear shape. Both the carbon and the nitrogen are sp hydridized which leaves them both with two p orbitals which overlap to form the two π bond in the triple bond. Industrially, the main methods for producing nitriles are ammoxidation and hydrocyanation. Both routes are green in the sense that they do not generate stoichiometric amounts of salts.Recommanded Product: 70291-62-2

2-Substituted 8,9-cycloalkathieno[3,2-e][1,2,4]triazolo[1,5-c]pyrimidin-5(6H)-ones was written by Chernyuk, O. M.;Tolmachova, V. S.;Vovk, M. V.. And the article was included in Ukrainskii Khimicheskii Zhurnal (Russian Edition) in 2009.Recommanded Product: 70291-62-2 This article mentions the following:

A series of cycloalkane-fused thieno[3.2-e][1,2,4]triazolo[1,5-c]pyrimidinones I [R1R2 = (CH2)3, (CH2)4, CH2CHMeCH2CH2, (CH2)5; R3 = Ph, 4-FC6H4, 2-furyl, 3-pyridyl, etc.] was synthesized by condensation of N-(cyanothienyl) carbamates II with carboxylic acids hydrazides R3CONHNH2. In the experiment, the researchers used many compounds, for example, 2-Amino-5,6-dihydro-4H-cyclopenta[b]thiophene-3-carbonitrile (cas: 70291-62-2Recommanded Product: 70291-62-2).

2-Amino-5,6-dihydro-4H-cyclopenta[b]thiophene-3-carbonitrile (cas: 70291-62-2) belongs to nitriles. The R-C-N bond angle in and nitrile is 180° which give a nitrile functional group a linear shape. Both the carbon and the nitrogen are sp hydridized which leaves them both with two p orbitals which overlap to form the two π bond in the triple bond. Industrially, the main methods for producing nitriles are ammoxidation and hydrocyanation. Both routes are green in the sense that they do not generate stoichiometric amounts of salts.Recommanded Product: 70291-62-2

Referemce:
Nitrile – Wikipedia,
Nitriles – Chemistry LibreTexts

Raju, Gollapalli Naga et al. published their research in Journal of Chemical and Pharmaceutical Research in 2017 | CAS: 70291-62-2

2-Amino-5,6-dihydro-4H-cyclopenta[b]thiophene-3-carbonitrile (cas: 70291-62-2) belongs to nitriles. Nitrile carbon shifts are in the range of 115–125 ppm whereas in isonitriles the shifts are around 155–165 ppm. Asymmetric bioreduction of nitriles is an attractive route to produce optically active nitriles as current metal-catalyzed hydrogenations tend to have low reactivity.COA of Formula: C8H8N2S

Design, characterisation and anticonvulsant evaluation of thienopyrimidinone derivatives synthesised by green chemistry was written by Raju, Gollapalli Naga;Sucharitha, Nallamothu;Navya, Nagandla;Nadendla, Rama Rao. And the article was included in Journal of Chemical and Pharmaceutical Research in 2017.COA of Formula: C8H8N2S This article mentions the following:

The synthesis and biol. evaluation of some novel thieno[2,3-d]pyrimidin-4(3H)one derivatives was aimed at creating a new scaffold. Considering the previous articles activity profile of thienopyrimidinone derivatives, fifteen new substituted thieno[2,3-d]pyrimidin-4(3H)one derivatives were synthesized by reacting different 2-amino-4,5-substituted thiophene-3-carbonitriles with appropriate aliphatic carboxylic acids in presence of phosphoryl trihloride and alumina by conventional synthesis as well as microwave assisted route. Structural characterizations of all the synthesized compounds were performed by using spectral and elemental anal. All the synthesized compounds were evaluated for their antimicrobial and anticonvulsant activities by using disk diffusion method and maximal electroshock induced seizures (MES) method. It was found that the compounds possessed potent antimicrobial and anticonvulsant activities. In the experiment, the researchers used many compounds, for example, 2-Amino-5,6-dihydro-4H-cyclopenta[b]thiophene-3-carbonitrile (cas: 70291-62-2COA of Formula: C8H8N2S).

2-Amino-5,6-dihydro-4H-cyclopenta[b]thiophene-3-carbonitrile (cas: 70291-62-2) belongs to nitriles. Nitrile carbon shifts are in the range of 115–125 ppm whereas in isonitriles the shifts are around 155–165 ppm. Asymmetric bioreduction of nitriles is an attractive route to produce optically active nitriles as current metal-catalyzed hydrogenations tend to have low reactivity.COA of Formula: C8H8N2S

Referemce:
Nitrile – Wikipedia,
Nitriles – Chemistry LibreTexts

Azuma, Kentaro et al. published their research in Photochemical & Photobiological Sciences in 2003 | CAS: 51473-74-6

7-(Diethylamino)-2-oxo-2H-chromene-3-carbonitrile (cas: 51473-74-6) belongs to nitriles. There has been no report on the microbial biosynthesis of nitriles and the physiological function of such enzymes, nor was it not even known whether aliphatic and aromatic nitriles are biological compounds or just petrochemicals. Some nitriles are manufactured by heating carboxylic acids with ammonia in the presence of catalysts. This process is used to make nitriles from natural fats and oils, the products being used as softening agents in synthetic rubbers, plastics, and textiles and for making amines.Recommanded Product: 7-(Diethylamino)-2-oxo-2H-chromene-3-carbonitrile

A study of the relationship between the chemical structures and the fluorescence quantum yields of coumarins, quinoxalinones and benzoxazinones for the development of sensitive fluorescent derivatization reagents was written by Azuma, Kentaro;Suzuki, Sachiko;Uchiyama, Seiichi;Kajiro, Toshi;Santa, Tomofumi;Imai, Kazuhiro. And the article was included in Photochemical & Photobiological Sciences in 2003.Recommanded Product: 7-(Diethylamino)-2-oxo-2H-chromene-3-carbonitrile This article mentions the following:

To develop new fluorescent derivatization reagents, we investigated the relationship between the chem. structures and the fluorescence quantum yields (Φf) of coumarins, quinoxalinones and benzoxadinones. Forty-six compounds were synthesized and their fluorescence spectra were measured in n-hexane, Et acetate, methanol and water. The energy levels of these compounds were calculated by combination of the semi-empirical AM1 and INDO/S (CI = all) methods. The ΔE(Tn(n,π*), S1(π,π*)) (the energy gap between the Tn(n,π*) and S1(π,π*) states) values were well correlated with the Φf values, which enables us to predict the Φf values from their chem. structures. Based on this relationship, 3-phenyl-7-N-piperazinoquinoxalin-2(1H)-one (PQ-Pz) and 7-(3-(S)-aminopyrrolidin-1-yl)-3-phenylquinoxalin-2-(1H)-one (PQ-APy) were developed as fluorescent derivatization reagents for carboxylic acids. The derivatives of the carboxylic acids with PQ-Pz and PQ-APy showed large Φf values even in polar solvents, suggesting that these reagents are suitable for the microanal. of biol. important carboxylic acids by reversed phase HPLC. In the experiment, the researchers used many compounds, for example, 7-(Diethylamino)-2-oxo-2H-chromene-3-carbonitrile (cas: 51473-74-6Recommanded Product: 7-(Diethylamino)-2-oxo-2H-chromene-3-carbonitrile).

7-(Diethylamino)-2-oxo-2H-chromene-3-carbonitrile (cas: 51473-74-6) belongs to nitriles. There has been no report on the microbial biosynthesis of nitriles and the physiological function of such enzymes, nor was it not even known whether aliphatic and aromatic nitriles are biological compounds or just petrochemicals. Some nitriles are manufactured by heating carboxylic acids with ammonia in the presence of catalysts. This process is used to make nitriles from natural fats and oils, the products being used as softening agents in synthetic rubbers, plastics, and textiles and for making amines.Recommanded Product: 7-(Diethylamino)-2-oxo-2H-chromene-3-carbonitrile

Referemce:
Nitrile – Wikipedia,
Nitriles – Chemistry LibreTexts

Zhao, Wei et al. published their research in ChemCatChem in 2016 | CAS: 53312-77-9

2-Amino-3-chlorobenzonitrile (cas: 53312-77-9) belongs to nitriles. There has been no report on the microbial biosynthesis of nitriles and the physiological function of such enzymes, nor was it not even known whether aliphatic and aromatic nitriles are biological compounds or just petrochemicals. Alkyl nitriles are sufficiently acidic to undergo deprotonation of the C-H bond adjacent to the CN group.Strong bases are required, such as lithium diisopropylamide and butyl lithium. The product is referred to as a nitrile anion. Category: nitriles-buliding-blocks

Quinazolinones from o-Aminobenzonitriles by One-Pot Sequential Selective Hydration/Condensation/Acceptorless Dehydrogenation Catalyzed by an Iridium Complex was written by Zhao, Wei;Liu, Pengcheng;Li, Feng. And the article was included in ChemCatChem in 2016.Category: nitriles-buliding-blocks This article mentions the following:

A new strategy for the direct synthesis of quinazolinones from o-aminobenzonitriles was proposed and accomplished. In the presence of [Cp*IrCl2]2 (Cp*=pentamethylcyclopentadienyl), a variety of desirable products was obtained easily through the one-pot sequential selective hydration/condensation/acceptorless dehydrogenation. This protocol was highly attractive because it used readily available starting materials, has a high atom efficiency, good to excellent yields, and minimal consumption of chems. and energy. Notably, this research exhibited new potential for the development of transition-metal-catalyzed one-pot sequential reactions that involve acceptorless dehydrogenation. In the experiment, the researchers used many compounds, for example, 2-Amino-3-chlorobenzonitrile (cas: 53312-77-9Category: nitriles-buliding-blocks).

2-Amino-3-chlorobenzonitrile (cas: 53312-77-9) belongs to nitriles. There has been no report on the microbial biosynthesis of nitriles and the physiological function of such enzymes, nor was it not even known whether aliphatic and aromatic nitriles are biological compounds or just petrochemicals. Alkyl nitriles are sufficiently acidic to undergo deprotonation of the C-H bond adjacent to the CN group.Strong bases are required, such as lithium diisopropylamide and butyl lithium. The product is referred to as a nitrile anion. Category: nitriles-buliding-blocks

Referemce:
Nitrile – Wikipedia,
Nitriles – Chemistry LibreTexts

You, Jingsong et al. published their research in Angewandte Chemie, International Edition in 2003 | CAS: 5351-07-5

2-(4-Methoxyphenyl)-2-methylpropanenitrile (cas: 5351-07-5) belongs to nitriles. Nitrile carbon shifts are in the range of 115–125 ppm whereas in isonitriles the shifts are around 155–165 ppm. In conventional organic reductions, nitrile is reduced by treatment with lithium aluminium hydride to the amine. Reduction to the imine followed by hydrolysis to the aldehyde takes place in the Stephen aldehyde synthesis, which uses stannous chloride in acid.SDS of cas: 5351-07-5

A general method for the direct α-arylation of nitriles with aryl chlorides was written by You, Jingsong;Verkade, John G.. And the article was included in Angewandte Chemie, International Edition in 2003.SDS of cas: 5351-07-5 This article mentions the following:

The long-standing challenge of developing a general method for the title methodol. is met for a broad range of aryl chlorides through the use of bicyclic P(iBuNCH2CH2)3N as bulky electron-rich ligand for palladium. In the experiment, the researchers used many compounds, for example, 2-(4-Methoxyphenyl)-2-methylpropanenitrile (cas: 5351-07-5SDS of cas: 5351-07-5).

2-(4-Methoxyphenyl)-2-methylpropanenitrile (cas: 5351-07-5) belongs to nitriles. Nitrile carbon shifts are in the range of 115–125 ppm whereas in isonitriles the shifts are around 155–165 ppm. In conventional organic reductions, nitrile is reduced by treatment with lithium aluminium hydride to the amine. Reduction to the imine followed by hydrolysis to the aldehyde takes place in the Stephen aldehyde synthesis, which uses stannous chloride in acid.SDS of cas: 5351-07-5

Referemce:
Nitrile – Wikipedia,
Nitriles – Chemistry LibreTexts

Broomfield, Lewis Marc et al. published their research in Advanced Synthesis & Catalysis in 2015 | CAS: 10282-32-3

4-(Benzylamino)benzonitrile (cas: 10282-32-3) belongs to nitriles. Nitrile carbon shifts are in the range of 115–125 ppm whereas in isonitriles the shifts are around 155–165 ppm. Nitriles are susceptible to hydrogenation over diverse metal catalysts. The reaction can afford either the primary amine (RCH2NH2) or the tertiary amine ((RCH2)3N), depending on conditions.Quality Control of 4-(Benzylamino)benzonitrile

Phosphino-amine (PN) Ligands for Rapid Catalyst Discovery in Ruthenium-Catalyzed Hydrogen-Borrowing Alkylation of Anilines: A Proof of Principle was written by Broomfield, Lewis Marc;Wu, Yichen;Martin, Eddy;Shafir, Alexandr. And the article was included in Advanced Synthesis & Catalysis in 2015.Quality Control of 4-(Benzylamino)benzonitrile This article mentions the following:

A general synthetic protocol for the synthesis of simple phosphino-amine (PN) ligands is described with 19 ligands being isolated in good yields. High-throughput ligand screening uncovered the success of two of these ligands for aromatic amine alkylations via ruthenium-catalyzed hydrogen borrowing reactions. The combination of N,N’-bis(diphenylphosphino)-N,N’-dimethylpropylenediamine with a ruthenium(II) source and potassium hydroxide (15 mol%) is the optimal system for selective monobenzylations of aromatic amines (method A). Over 70% isolated yields have been achieved for the formation of 14 secondary aromatic amines under mild reaction conditions (120° and 1.05 equivalent of benzyl alc.). On the other hand, N,N-bis(diphenylphosphino)-isopropylamine was the ligand utilized for both selective monomethylation and monoethylation reactions of aromatic amines (method B). Here the alc. is charged as both the reaction medium and substrate and 9 examples are disclosed with all isolated yields exceeding 70%. These methods have been applied to the synthesis of important synthetic building blocks based on aminoferrocene. In the experiment, the researchers used many compounds, for example, 4-(Benzylamino)benzonitrile (cas: 10282-32-3Quality Control of 4-(Benzylamino)benzonitrile).

4-(Benzylamino)benzonitrile (cas: 10282-32-3) belongs to nitriles. Nitrile carbon shifts are in the range of 115–125 ppm whereas in isonitriles the shifts are around 155–165 ppm. Nitriles are susceptible to hydrogenation over diverse metal catalysts. The reaction can afford either the primary amine (RCH2NH2) or the tertiary amine ((RCH2)3N), depending on conditions.Quality Control of 4-(Benzylamino)benzonitrile

Referemce:
Nitrile – Wikipedia,
Nitriles – Chemistry LibreTexts

Huang, Zhuo-Bin et al. published their research in Asian Journal of Organic Chemistry in 2020 | CAS: 36057-44-0

4-methoxypicolinonitrile (cas: 36057-44-0) belongs to nitriles. Nitrile carbon shifts are in the range of 115–125 ppm whereas in isonitriles the shifts are around 155–165 ppm. Industrially, the main methods for producing nitriles are ammoxidation and hydrocyanation. Both routes are green in the sense that they do not generate stoichiometric amounts of salts.Product Details of 36057-44-0

Selectively Oxidative Thiolysis of Nitriles into Primary Thioamides and Insecticidal Application was written by Huang, Zhuo-Bin;Guo, Xue-Ying;Huang, Zi-Hao;Li, Ming-Hua;Dong, Shou-Cheng;Tang, Ri-Yuan. And the article was included in Asian Journal of Organic Chemistry in 2020.Product Details of 36057-44-0 This article mentions the following:

Primary thioamides were useful building blocks for drug and insecticide development, therefore an environmentally benign synthesis of primary thioamides was desired. An oxidative thiolysis for the selective transformation of nitriles into primary thioamides using elemental sulfur or thiuram in the presence of K2S2O8 in DMF/H2O was discussed. This practical method enables access to a wide range of synthetically and pharmaceutically useful primary thioamides. Advantages of this reaction include transition-metal-free and base-free reaction conditions, use of an environmentally benign solvent (DMF/H2O) system, the use of non-toxic elemental sulfur or thiuram as the sulfur sources, and good functional groups tolerances with excellent selectivity. Furthermore, the insecticide Fipronil was also converted to the corresponding thioamide and maintains excellent bioactivity against P. xylostella. The LC50 value of Fipronil thioamide was 1.25 mg/L. In the experiment, the researchers used many compounds, for example, 4-methoxypicolinonitrile (cas: 36057-44-0Product Details of 36057-44-0).

4-methoxypicolinonitrile (cas: 36057-44-0) belongs to nitriles. Nitrile carbon shifts are in the range of 115–125 ppm whereas in isonitriles the shifts are around 155–165 ppm. Industrially, the main methods for producing nitriles are ammoxidation and hydrocyanation. Both routes are green in the sense that they do not generate stoichiometric amounts of salts.Product Details of 36057-44-0

Referemce:
Nitrile – Wikipedia,
Nitriles – Chemistry LibreTexts

Claffey, Michelle M. et al. published their research in Journal of Medicinal Chemistry in 2012 | CAS: 104798-53-0

2-Fluoro-5-hydroxybenzenecarbonitrile (cas: 104798-53-0) belongs to nitriles. Nitriles are polar, as indicated by high dipole moments. As liquids, they have high relative permittivities, often in the 30s. Nitrile groups in organic compounds can undergo a variety of reactions depending on the reactants or conditions. A nitrile group can be hydrolyzed, reduced, or ejected from a molecule as a cyanide ion.Recommanded Product: 2-Fluoro-5-hydroxybenzenecarbonitrile

Application of Structure-Based Drug Design and Parallel Chemistry to Identify Selective, Brain Penetrant, In Vivo Active Phosphodiesterase 9A Inhibitors was written by Claffey, Michelle M.;Helal, Christopher J.;Verhoest, Patrick R.;Kang, Zhijun;Bundesmann, Mark W.;Hou, Xinjun;Lui, Shenping;Kleiman, Robin J.;Vanase-Frawley, Michelle;Schmidt, Anne W.;Menniti, Frank;Schmidt, Christopher J.;Hoffman, William E.;Hajos, Mihaly;McDowell, Laura;O’Connor, Rebecca E.;MacDougall-Murphy, Mary;Fonseca, Kari R.;Becker, Stacey L.;Nelson, Frederick R.;Liras, Spiros. And the article was included in Journal of Medicinal Chemistry in 2012.Recommanded Product: 2-Fluoro-5-hydroxybenzenecarbonitrile This article mentions the following:

Phosphodiesterase 9A inhibitors have shown activity in preclin. models of cognition with potential application as novel therapies for treating Alzheimer’s disease. Our clin. candidate, PF-04447943 (2), demonstrated acceptable CNS permeability in rats with modest asymmetry between central and peripheral compartments (free brain/free plasma = 0.32; CSF/free plasma = 0.19) yet had physicochem. properties outside the range associated with traditional CNS drugs. To address the potential risk of restricted CNS penetration with 2 in human clin. trials, we sought to identify a preclin. candidate with no asymmetry in rat brain penetration and that could advance into development. Merging the medicinal chem. strategies of structure-based design with parallel chem., a novel series of PDE9A inhibitors was identified that showed improved selectivity over PDE1C. Optimization afforded preclin. candidate 19 that demonstrated free brain/free plasma ≥1 in rat and reduced microsomal clearance along with the ability to increase cyclic guanosine monophosphosphate levels in rat CSF. In the experiment, the researchers used many compounds, for example, 2-Fluoro-5-hydroxybenzenecarbonitrile (cas: 104798-53-0Recommanded Product: 2-Fluoro-5-hydroxybenzenecarbonitrile).

2-Fluoro-5-hydroxybenzenecarbonitrile (cas: 104798-53-0) belongs to nitriles. Nitriles are polar, as indicated by high dipole moments. As liquids, they have high relative permittivities, often in the 30s. Nitrile groups in organic compounds can undergo a variety of reactions depending on the reactants or conditions. A nitrile group can be hydrolyzed, reduced, or ejected from a molecule as a cyanide ion.Recommanded Product: 2-Fluoro-5-hydroxybenzenecarbonitrile

Referemce:
Nitrile – Wikipedia,
Nitriles – Chemistry LibreTexts